Merge remote-tracking branch 'upstream/master'

infra/README.md

@@ -53,7 +53,7 @@ The typical cycle for editing a recipe is:
 
 1. Make your edits.
 2. Run `build/scripts/slave/recipes.py simulation_test train flutter` to update expected files  (remove the flutter if you need to do a global update).
-3. Run `build/scripts/slave/recipes.py run flutter/flutter` (or flutter/engine) if something was strange during training and you need to run it locally.
+3. Run `build/scripts/tools/run_recipe.py flutter/flutter` (or flutter/engine) if something was strange during training and you need to run it locally.
 4. Upload the patch (`git commit`, `git cl upload`) and send it to someone in the `recipes/flutter/OWNERS` file for review.
 
 ## Editing the client.flutter buildbot master

packages/flutter/lib/animation.dart

@@ -4,7 +4,7 @@
 
 /// The Flutter animation system.
 ///
-/// See [https://flutter.io/animations/] for an overview.
+/// See <https://flutter.io/animations/> for an overview.
 ///
 /// This library depends only on core Dart libraries and the `newton` package.
 library animation;

packages/flutter/lib/shell.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// Manages connections with embedder-provided services.
+library shell;
+
+import 'dart:ui' as ui;
+
+import 'package:mojo/application.dart';
+import 'package:mojo/bindings.dart' as bindings;
+import 'package:mojo/core.dart' as core;
+import 'package:mojo/mojo/service_provider.mojom.dart' as mojom;
+import 'package:mojo/mojo/shell.mojom.dart' as mojom;
+
+/// A replacement for shell.connectToService.  Implementations should return true
+/// if they handled the request, or false if the request should fall through
+/// to the default requestService.
+typedef bool OverrideConnectToService(String url, Object proxy);
+
+/// Manages connections with embedder-provided services.
+class MojoShell {
+  MojoShell() {
+    assert(_instance == null);
+    _instance = this;
+  }
+
+  /// The unique instance of this class.
+  static MojoShell get instance => _instance;
+  static MojoShell _instance;
+
+  static mojom.ShellProxy _initShellProxy() {
+    core.MojoHandle shellHandle = new core.MojoHandle(ui.takeShellProxyHandle());
+    if (!shellHandle.isValid)
+      return null;
+    return new mojom.ShellProxy.fromHandle(shellHandle);
+  }
+  final mojom.Shell _shell = _initShellProxy()?.ptr;
+
+  static ApplicationConnection _initEmbedderConnection() {
+    core.MojoHandle servicesHandle = new core.MojoHandle(ui.takeServicesProvidedByEmbedder());
+    core.MojoHandle exposedServicesHandle = new core.MojoHandle(ui.takeServicesProvidedToEmbedder());
+    if (!servicesHandle.isValid || !exposedServicesHandle.isValid)
+      return null;
+    mojom.ServiceProviderProxy services = new mojom.ServiceProviderProxy.fromHandle(servicesHandle);
+    mojom.ServiceProviderStub exposedServices = new mojom.ServiceProviderStub.fromHandle(exposedServicesHandle);
+    return new ApplicationConnection(exposedServices, services);
+  }
+  final ApplicationConnection _embedderConnection = _initEmbedderConnection();
+
+  /// Attempts to connect to an application via the Mojo shell.
+  ApplicationConnection connectToApplication(String url) {
+    if (_shell == null)
+      return null;
+    mojom.ServiceProviderProxy services = new mojom.ServiceProviderProxy.unbound();
+    mojom.ServiceProviderStub exposedServices = new mojom.ServiceProviderStub.unbound();
+    _shell.connectToApplication(url, services, exposedServices);
+    return new ApplicationConnection(exposedServices, services);
+  }
+
+  /// Set this to intercept calls to [connectToService()] and supply an
+  /// alternative implementation of a service (for example, a mock for testing).
+  OverrideConnectToService overrideConnectToService;
+
+  /// Attempts to connect to a service implementing the interface for the given proxy.
+  /// If an application URL is specified, the service will be requested from that application.
+  /// Otherwise, it will be requested from the embedder (the Flutter engine).
+  void connectToService(String url, bindings.ProxyBase proxy) {
+    if (overrideConnectToService != null && overrideConnectToService(url, proxy))
+      return;
+    if (url == null || _shell == null) {
+      // If the application URL is null, it means the service to connect
+      // to is one provided by the embedder.
+      // If the applircation URL isn't null but there's no shell, then
+      // ask the embedder in case it provides it. (For example, if you're
+      // running on Android without the Mojo shell, then you can obtain
+      // the media service from the embedder directly, instead of having
+      // to ask the media application for it.)
+      // This makes it easier to write an application that works both
+      // with and without a Mojo environment.
+      _embedderConnection?.requestService(proxy);
+      return;
+    }
+    mojom.ServiceProviderProxy services = new mojom.ServiceProviderProxy.unbound();
+    _shell.connectToApplication(url, services, null);
+    core.MojoMessagePipe pipe = new core.MojoMessagePipe();
+    proxy.impl.bind(pipe.endpoints[0]);
+    services.ptr.connectToService(proxy.serviceName, pipe.endpoints[1]);
+    services.close();
+  }
+
+  /// Registers a service to expose to the embedder.
+  void provideService(String interfaceName, ServiceFactory factory) {
+    _embedderConnection?.provideService(interfaceName, factory);
+  }
+}
+
+/// The singleton object that manages connections with embedder-provided services.
+MojoShell get shell => MojoShell.instance;

packages/flutter/lib/src/material/app.dart

@@ -2,13 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:async';
-import 'dart:ui' as ui show WindowPadding, window;
-
 import 'package:flutter/rendering.dart';
-import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
+import 'colors.dart';
 import 'page.dart';
 import 'theme.dart';
 
@@ -21,204 +18,75 @@ const TextStyle _errorTextStyle = const TextStyle(
   fontWeight: FontWeight.w900,
   textAlign: TextAlign.right,
   decoration: TextDecoration.underline,
-  decorationColor: const Color(0xFFFF00),
+  decorationColor: const Color(0xFFFFFF00),
   decorationStyle: TextDecorationStyle.double
 );
 
-AssetBundle _initDefaultBundle() {
-  if (rootBundle != null)
-    return rootBundle;
-  return new NetworkAssetBundle(Uri.base);
-}
-
-final AssetBundle _defaultBundle = _initDefaultBundle();
-
-class RouteArguments {
-  const RouteArguments({ this.context });
-  final BuildContext context;
-}
-typedef Widget RouteBuilder(RouteArguments args);
-
-typedef Future<LocaleQueryData> LocaleChangedCallback(Locale locale);
 
-class MaterialApp extends StatefulComponent {
+class MaterialApp extends WidgetsApp {
   MaterialApp({
     Key key,
-    this.title,
-    this.theme,
-    this.routes: const <String, RouteBuilder>{},
-    this.onGenerateRoute,
-    this.onLocaleChanged,
+    String title,
+    ThemeData theme,
+    Map<String, RouteBuilder> routes: const <String, RouteBuilder>{},
+    RouteFactory onGenerateRoute,
+    LocaleChangedCallback onLocaleChanged,
     this.debugShowMaterialGrid: false,
-    this.showPerformanceOverlay: false,
-    this.showSemanticsDebugger: false,
-    this.debugShowCheckedModeBanner: true
-  }) : super(key: key) {
-    assert(routes != null);
-    assert(routes.containsKey(Navigator.defaultRouteName) || onGenerateRoute != null);
+    bool showPerformanceOverlay: false,
+    bool showSemanticsDebugger: false,
+    bool debugShowCheckedModeBanner: true
+  }) : theme = theme,
+       super(
+    key: key,
+    title: title,
+    textStyle: _errorTextStyle,
+    color: theme?.primaryColor ?? Colors.blue[500], // blue[500] is the primary color of the default theme
+    routes: routes,
+    onGenerateRoute: (RouteSettings settings) {
+      RouteBuilder builder = routes[settings.name];
+      if (builder != null) {
+        return new MaterialPageRoute(
+          builder: (BuildContext context) {
+            return builder(new RouteArguments(context: context));
+          },
+          settings: settings
+        );
+      }
+      if (onGenerateRoute != null)
+        return onGenerateRoute(settings);
+      return null;
+    },
+    onLocaleChanged: onLocaleChanged,
+    showPerformanceOverlay: showPerformanceOverlay,
+    showSemanticsDebugger: showSemanticsDebugger,
+    debugShowCheckedModeBanner: debugShowCheckedModeBanner
+  ) {
     assert(debugShowMaterialGrid != null);
-    assert(showPerformanceOverlay != null);
-    assert(showSemanticsDebugger != null);
   }
 
-  /// A one-line description of this app for use in the window manager.
-  final String title;
-
   /// The colors to use for the application's widgets.
   final ThemeData theme;
 
-  /// The default table of routes for the application. When the
-  /// [Navigator] is given a named route, the name will be looked up
-  /// in this table first. If the name is not available, then
-  /// [onGenerateRoute] will be called instead.
-  final Map<String, RouteBuilder> routes;
-
-  /// The route generator callback used when the app is navigated to a
-  /// named route but the name is not in the [routes] table.
-  final RouteFactory onGenerateRoute;
-
-  /// Callback that is invoked when the operating system changes the
-  /// current locale.
-  final LocaleChangedCallback onLocaleChanged;
-
   /// Turns on a [GridPaper] overlay that paints a baseline grid
   /// Material apps:
   /// https://www.google.com/design/spec/layout/metrics-keylines.html
   /// Only available in checked mode.
   final bool debugShowMaterialGrid;
 
-  /// Turns on a performance overlay.
-  /// https://flutter.io/debugging/#performanceoverlay
-  final bool showPerformanceOverlay;
-
-  /// Turns on an overlay that shows the accessibility information
-  /// reported by the framework.
-  final bool showSemanticsDebugger;
-
-  /// Turns on a "SLOW MODE" little banner in checked mode to indicate
-  /// that the app is in checked mode. This is on by default (in
-  /// checked mode), to turn it off, set the constructor argument to
-  /// false. In release mode this has no effect.
-  ///
-  /// To get this banner in your application if you're not using
-  /// MaterialApp, include a [CheckedModeBanner] widget in your app.
-  ///
-  /// This banner is intended to avoid people complaining that your
-  /// app is slow when it's in checked mode. In checked mode, Flutter
-  /// enables a large number of expensive diagnostics to aid in
-  /// development, and so performance in checked mode is not
-  /// representative of what will happen in release mode.
-  final bool debugShowCheckedModeBanner;
-
   _MaterialAppState createState() => new _MaterialAppState();
 }
 
-EdgeDims _getPadding(ui.WindowPadding padding) {
-  return new EdgeDims.TRBL(padding.top, padding.right, padding.bottom, padding.left);
-}
-
-class _MaterialAppState extends State<MaterialApp> implements BindingObserver {
-
-  GlobalObjectKey _navigator;
-
-  LocaleQueryData _localeData;
-
-  void initState() {
-    super.initState();
-    _navigator = new GlobalObjectKey(this);
-    didChangeLocale(ui.window.locale);
-    WidgetFlutterBinding.instance.addObserver(this);
-  }
-
-  void dispose() {
-    WidgetFlutterBinding.instance.removeObserver(this);
-    super.dispose();
-  }
-
-  bool didPopRoute() {
-    assert(mounted);
-    NavigatorState navigator = _navigator.currentState;
-    assert(navigator != null);
-    bool result = false;
-    navigator.openTransaction((NavigatorTransaction transaction) {
-      result = transaction.pop();
-    });
-    return result;
-  }
-
-  void didChangeMetrics() {
-    setState(() {
-      // The properties of ui.window have changed. We use them in our build
-      // function, so we need setState(), but we don't cache anything locally.
-    });
-  }
-
-  void didChangeLocale(Locale locale) {
-    if (config.onLocaleChanged != null) {
-      config.onLocaleChanged(locale).then((LocaleQueryData data) {
-        if (mounted)
-          setState(() { _localeData = data; });
-      });
-    }
-  }
-
-  void didChangeAppLifecycleState(AppLifecycleState state) { }
+class _MaterialAppState extends WidgetsAppState<MaterialApp> {
 
   final HeroController _heroController = new HeroController();
-
-  Route _generateRoute(RouteSettings settings) {
-    RouteBuilder builder = config.routes[settings.name];
-    if (builder != null) {
-      return new MaterialPageRoute(
-        builder: (BuildContext context) {
-          return builder(new RouteArguments(context: context));
-        },
-        settings: settings
-      );
-    }
-    if (config.onGenerateRoute != null)
-      return config.onGenerateRoute(settings);
-    return null;
-  }
+  NavigatorObserver get navigatorObserver => _heroController;
 
   Widget build(BuildContext context) {
-    if (config.onLocaleChanged != null && _localeData == null) {
-      // If the app expects a locale but we don't yet know the locale, then
-      // don't build the widgets now.
-      return new Container();
-    }
-
     ThemeData theme = config.theme ?? new ThemeData.fallback();
-    Widget result = new MediaQuery(
-      data: new MediaQueryData(
-        size: ui.window.size,
-        devicePixelRatio: ui.window.devicePixelRatio,
-        padding: _getPadding(ui.window.padding)
-      ),
-      child: new LocaleQuery(
-        data: _localeData,
-        child: new AnimatedTheme(
-          data: theme,
-          duration: kThemeAnimationDuration,
-          child: new DefaultTextStyle(
-            style: _errorTextStyle,
-            child: new AssetVendor(
-              bundle: _defaultBundle,
-              devicePixelRatio: ui.window.devicePixelRatio,
-              child: new Title(
-                title: config.title,
-                color: theme.primaryColor,
-                child: new Navigator(
-                  key: _navigator,
-                  initialRoute: ui.window.defaultRouteName,
-                  onGenerateRoute: _generateRoute,
-                  observer: _heroController
-                )
-              )
-            )
-          )
-        )
-      )
+    Widget result = new AnimatedTheme(
+      data: theme,
+      duration: kThemeAnimationDuration,
+      child: super.build(context)
     );
     assert(() {
       if (config.debugShowMaterialGrid) {
@@ -232,27 +100,6 @@ class _MaterialAppState extends State<MaterialApp> implements BindingObserver {
       }
       return true;
     });
-    if (config.showPerformanceOverlay) {
-      result = new Stack(
-        children: <Widget>[
-          result,
-          new Positioned(bottom: 0.0, left: 0.0, right: 0.0, child: new PerformanceOverlay.allEnabled()),
-        ]
-      );
-    }
-    if (config.showSemanticsDebugger) {
-      result = new SemanticsDebugger(
-        child: result
-      );
-    }
-    assert(() {
-      if (config.debugShowCheckedModeBanner) {
-        result = new CheckedModeBanner(
-          child: result
-        );
-      }
-      return true;
-    });
     return result;
   }
 

packages/flutter/lib/src/material/bottom_sheet.dart

@@ -80,7 +80,7 @@ class _BottomSheetState extends State<BottomSheet> {
       onVerticalDragUpdate: _handleDragUpdate,
       onVerticalDragEnd: _handleDragEnd,
       child: new Material(
-          key: _childKey,
+        key: _childKey,
         child: config.builder(context)
       )
     );

packages/flutter/lib/src/material/date_picker.dart

@@ -10,6 +10,7 @@ import 'package:intl/date_symbols.dart';
 import 'package:intl/intl.dart';
 
 import 'colors.dart';
+import 'debug.dart';
 import 'ink_well.dart';
 import 'theme.dart';
 import 'typography.dart';
@@ -403,6 +404,7 @@ class _YearPickerState extends State<YearPicker> {
   }
 
   Widget build(BuildContext context) {
+    assert(debugCheckHasMaterial(context));
     return new ScrollableLazyList(
       itemExtent: _itemExtent,
       itemCount: config.lastDate.year - config.firstDate.year + 1,

packages/flutter/lib/src/material/debug.dart

@@ -12,8 +12,17 @@ bool debugCheckHasMaterial(BuildContext context) {
     if (context.widget is! Material && context.ancestorWidgetOfExactType(Material) == null) {
       Element element = context;
       throw new WidgetError(
-        'Missing Material widget.',
-        '${context.widget} needs to be placed inside a Material widget. Ownership chain:\n${element.debugGetOwnershipChain(10)}'
+        'No Material widget found.\n'
+        '${context.widget.runtimeType} widgets require a Material widget ancestor.\n'
+        'In material design, most widgets are conceptually "printed" on a sheet of material. In Flutter\'s material library, '
+        'that material is represented by the Material widget. It is the Material widget that renders ink splashes, for instance. '
+        'Because of this, many material library widgets require that there be a Material widget in the tree above them.\n'
+        'To introduce a Material widget, you can either directly include one, or use a widget that contains Material itself, '
+        'such as a Card, Dialog, Drawer, or Scaffold.\n'
+        'The specific widget that could not find a Material ancestor was:\n'
+        '  ${context.widget}'
+        'The ownership chain for the affected widget is:\n'
+        '  ${element.debugGetOwnershipChain(10)}'
       );
     }
     return true;
@@ -27,8 +36,12 @@ bool debugCheckHasScaffold(BuildContext context) {
     if (Scaffold.of(context) == null) {
       Element element = context;
       throw new WidgetError(
-        'Missing Scaffold widget.',
-        '${context.widget} needs to be placed inside the body of a Scaffold widget. Ownership chain:\n${element.debugGetOwnershipChain(10)}'
+        'No Scaffold widget found.\n'
+        '${context.widget.runtimeType} widgets require a Scaffold widget ancestor.\n'
+        'The specific widget that could not find a Scaffold ancestor was:\n'
+        '  ${context.widget}'
+        'The ownership chain for the affected widget is:\n'
+        '  ${element.debugGetOwnershipChain(10)}'
       );
     }
     return true;

packages/flutter/lib/src/material/drawer_item.dart

@@ -5,6 +5,7 @@
 import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
+import 'debug.dart';
 import 'icon.dart';
 import 'icons.dart';
 import 'ink_well.dart';
@@ -55,6 +56,7 @@ class DrawerItem extends StatelessComponent {
   }
 
   Widget build(BuildContext context) {
+    assert(debugCheckHasMaterial(context));
     ThemeData themeData = Theme.of(context);
 
     List<Widget> children = <Widget>[];

packages/flutter/lib/src/material/dropdown.dart

@@ -182,6 +182,7 @@ class _DropDownRoute<T> extends PopupRoute<_DropDownRouteResult<T>> {
 
   ModalPosition getPosition(BuildContext context) {
     RenderBox overlayBox = Overlay.of(context).context.findRenderObject();
+    assert(overlayBox != null); // can't be null; routes get inserted by Navigator which has its own Overlay
     Size overlaySize = overlayBox.size;
     RelativeRect menuRect = new RelativeRect.fromSize(rect, overlaySize);
     return new ModalPosition(

packages/flutter/lib/src/material/icon_button.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter/widgets.dart';
 
+import 'debug.dart';
 import 'icon.dart';
 import 'icons.dart';
 import 'ink_well.dart';
@@ -53,6 +54,7 @@ class IconButton extends StatelessComponent {
   final String tooltip;
 
   Widget build(BuildContext context) {
+    assert(debugCheckHasMaterial(context));
     Widget result = new Padding(
       padding: const EdgeDims.all(8.0),
       child: new Icon(

packages/flutter/lib/src/material/ink_well.dart

@@ -12,6 +12,18 @@ import 'debug.dart';
 import 'material.dart';
 import 'theme.dart';
 
+/// An area of a Material that responds to touch. Has a configurable shape and
+/// can be configured to clip splashes that extend outside its bounds or not.
+///
+/// For a variant of this widget that is specialised for rectangular areas that
+/// always clip splashes, see [InkWell].
+///
+/// Must have an ancestor [Material] widget in which to cause ink reactions.
+///
+/// If a Widget uses this class directly, it should include the following line
+/// at the top of its [build] function to call [debugCheckHasMaterial]:
+///
+///     assert(debugCheckHasMaterial(context));
 class InkResponse extends StatefulComponent {
   InkResponse({
     Key key,
@@ -156,9 +168,14 @@ class _InkResponseState<T extends InkResponse> extends State<T> {
 
 }
 
-/// An area of a Material that responds to touch.
+/// A rectangular area of a Material that responds to touch.
+///
+/// Must have an ancestor [Material] widget in which to cause ink reactions.
+///
+/// If a Widget uses this class directly, it should include the following line
+/// at the top of its [build] function to call [debugCheckHasMaterial]:
 ///
-/// Must have an ancestor Material widget in which to cause ink reactions.
+///     assert(debugCheckHasMaterial(context));
 class InkWell extends InkResponse {
   InkWell({
     Key key,

packages/flutter/lib/src/material/list_item.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter/widgets.dart';
 
+import 'debug.dart';
 import 'ink_well.dart';
 import 'theme.dart';
 
@@ -85,6 +86,7 @@ class ListItem extends StatelessComponent {
   }
 
   Widget build(BuildContext context) {
+    assert(debugCheckHasMaterial(context));
     final bool isTwoLine = !isThreeLine && secondary != null;
     final bool isOneLine = !isThreeLine && !isTwoLine;
     double itemHeight;

packages/flutter/lib/src/material/scaffold.dart

@@ -39,9 +39,8 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
 
   final EdgeDims padding;
 
-  void performLayout(Size size, BoxConstraints constraints) {
-
-    BoxConstraints looseConstraints = constraints.loosen();
+  void performLayout(Size size) {
+    BoxConstraints looseConstraints = new BoxConstraints.loose(size);
 
     // This part of the layout has the same effect as putting the toolbar and
     // body in a column and making the body flexible. What's different is that

packages/flutter/lib/src/material/tabs.dart

@@ -10,6 +10,7 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
+import 'debug.dart';
 import 'icon.dart';
 import 'icons.dart';
 import 'icon_theme.dart';
@@ -330,6 +331,7 @@ class _Tab extends StatelessComponent {
   }
 
   Widget build(BuildContext context) {
+    assert(debugCheckHasMaterial(context));
     Widget labelContent;
     if (label.icon == null && label.iconBuilder == null) {
       labelContent = _buildLabelText();

packages/flutter/lib/src/material/tooltip.dart

@@ -46,6 +46,7 @@ class Tooltip extends StatefulComponent {
     assert(preferBelow != null);
     assert(fadeDuration != null);
     assert(showDuration != null);
+    assert(child != null);
   }
 
   final String message;
@@ -150,7 +151,7 @@ class _TooltipState extends State<Tooltip> {
           preferBelow: config.preferBelow
         );
       });
-      Overlay.of(context).insert(_entry);
+      Overlay.of(context, debugRequiredFor: config).insert(_entry);
     }
     _timer?.cancel();
     if (_controller.status != AnimationStatus.completed) {
@@ -175,7 +176,7 @@ class _TooltipState extends State<Tooltip> {
   }
 
   Widget build(BuildContext context) {
-    assert(Overlay.of(context) != null);
+    assert(Overlay.of(context, debugRequiredFor: config) != null);
     return new GestureDetector(
       behavior: HitTestBehavior.opaque,
       onLongPress: showTooltip,

packages/flutter/lib/src/painting/text_style.dart

@@ -31,22 +31,22 @@ class TextStyle {
   /// The color to use when painting the text.
   final Color color;
 
-  /// The name of the font to use when painting the text.
+  /// The name of the font to use when painting the text (e.g., Roboto).
   final String fontFamily;
 
   /// The size of gyphs (in logical pixels) to use when painting the text.
   final double fontSize;
 
-  /// The font weight to use when painting the text.
+  /// The typeface thickness to use when painting the text (e.g., bold).
   final FontWeight fontWeight;
 
-  /// The font style to use when painting the text.
+  /// The typeface variant to use when drawing the letters (e.g., italics).
   final FontStyle fontStyle;
 
-  /// The amount of space to add between each letter.
+  /// The amount of space (in logical pixels) to add between each letter.
   final double letterSpacing;
 
-  /// The amount of space to add at each sequence of white-space (i.e. between each word).
+  /// The amount of space (in logical pixels) to add at each sequence of white-space (i.e. between each word).
   final double wordSpacing;
 
   /// How the text should be aligned (applies only to the outermost
@@ -59,13 +59,13 @@ class TextStyle {
   /// The distance between the text baselines, as a multiple of the font size.
   final double height;
 
-  /// The decorations to paint near the text.
+  /// The decorations to paint near the text (e.g., an underline).
   final TextDecoration decoration;
 
   /// The color in which to paint the text decorations.
   final Color decorationColor;
 
-  /// The style in which to paint the text decorations.
+  /// The style in which to paint the text decorations (e.g., dashed).
   final TextDecorationStyle decorationStyle;
 
   /// Returns a new text style that matches this text style but with the given

packages/flutter/lib/src/rendering/binding.dart

@@ -19,7 +19,7 @@ import 'semantics.dart';
 export 'package:flutter/gestures.dart' show HitTestResult;
 
 /// The glue between the render tree and the Flutter engine.
-abstract class Renderer extends Object with Scheduler, MojoShell
+abstract class Renderer extends Object with Scheduler, Services
   implements HitTestable {
 
   void initInstances() {
@@ -67,7 +67,7 @@ abstract class Renderer extends Object with Scheduler, MojoShell
 
   void initSemantics() {
     SemanticsNode.onSemanticsEnabled = renderView.scheduleInitialSemantics;
-    provideService(mojom.SemanticsServer.serviceName, (core.MojoMessagePipeEndpoint endpoint) {
+    shell.provideService(mojom.SemanticsServer.serviceName, (core.MojoMessagePipeEndpoint endpoint) {
       mojom.SemanticsServerStub server = new mojom.SemanticsServerStub.fromEndpoint(endpoint);
       server.impl = new SemanticsServer();
     });
@@ -116,7 +116,7 @@ void debugDumpSemanticsTree() {
 
 /// A concrete binding for applications that use the Rendering framework
 /// directly. This is the glue that binds the framework to the Flutter engine.
-class RenderingFlutterBinding extends BindingBase with Scheduler, Gesturer, MojoShell, Renderer {
+class RenderingFlutterBinding extends BindingBase with Scheduler, Gesturer, Services, Renderer {
   RenderingFlutterBinding({ RenderBox root }) {
     assert(renderView != null);
     renderView.child = root;

packages/flutter/lib/src/rendering/child_view.dart

@@ -20,7 +20,7 @@ import 'object.dart';
 mojom.ViewProxy _initViewProxy() {
   int viewHandle = ui.takeViewHandle();
   assert(() {
-    if (viewHandle == 0)
+    if (viewHandle == core.MojoHandle.INVALID)
       debugPrint('Child view are supported only when running in Mojo shell.');
     return true;
   });
@@ -35,8 +35,12 @@ mojom.ViewProxy _initViewProxy() {
 final mojom.ViewProxy _viewProxy = _initViewProxy();
 final mojom.View _view = _viewProxy?.ptr;
 
+/// (mojo-only) A connection with a child view.
+///
+/// Used with the [ChildView] widget to display a child view.
 class ChildViewConnection {
-  ChildViewConnection({ this.url }) {
+  /// Establishes a connection to the app at the given URL.
+  ChildViewConnection({ String url }) {
     mojom.ViewProviderProxy viewProvider = new mojom.ViewProviderProxy.unbound();
     shell.connectToService(url, viewProvider);
     mojom.ServiceProviderProxy incomingServices = new mojom.ServiceProviderProxy.unbound();
@@ -47,8 +51,16 @@ class ChildViewConnection {
     _connection = new ApplicationConnection(outgoingServices, incomingServices);
   }
 
-  final String url;
+  /// Wraps an already-established connection ot a child app.
+  ChildViewConnection.fromViewOwner({
+    mojom.ViewOwnerProxy viewOwner,
+    ApplicationConnection connection
+  }) : _connection = connection, _viewOwner = viewOwner;
 
+  /// The underlying application connection to the child app.
+  ///
+  /// Useful for requesting services from the child app and for providing
+  /// services to the child app.
   ApplicationConnection get connection => _connection;
   ApplicationConnection _connection;
 
@@ -120,18 +132,16 @@ class ChildViewConnection {
       ..devicePixelRatio = scale;
     return (await _view.layoutChild(_viewKey, layoutParams)).info;
   }
-
-  String toString() {
-    return '$runtimeType(url: $url)';
-  }
 }
 
+/// (mojo-only) A view of a child application.
 class RenderChildView extends RenderBox {
   RenderChildView({
     ChildViewConnection child,
     double scale
   }) : _child = child, _scale = scale;
 
+  /// The child to display.
   ChildViewConnection get child => _child;
   ChildViewConnection _child;
   void set child (ChildViewConnection value) {
@@ -150,6 +160,7 @@ class RenderChildView extends RenderBox {
     }
   }
 
+  /// The device pixel ratio to provide the child.
   double get scale => _scale;
   double _scale;
   void set scale (double value) {

packages/flutter/lib/src/rendering/custom_layout.dart

@@ -103,7 +103,7 @@ abstract class MultiChildLayoutDelegate {
     return '${childParentData.id}: $child';
   }
 
-  void _callPerformLayout(Size size, BoxConstraints constraints, RenderBox firstChild) {
+  void _callPerformLayout(Size size, RenderBox firstChild) {
     // A particular layout delegate could be called reentrantly, e.g. if it used
     // by both a parent and a child. So, we must restore the _idToChild map when
     // we return.
@@ -138,7 +138,7 @@ abstract class MultiChildLayoutDelegate {
         });
         child = childParentData.nextSibling;
       }
-      performLayout(size, constraints);
+      performLayout(size);
       assert(() {
         if (_debugChildrenNeedingLayout.isNotEmpty) {
           if (_debugChildrenNeedingLayout.length > 1) {
@@ -176,11 +176,10 @@ abstract class MultiChildLayoutDelegate {
   /// possible given the constraints.
   Size getSize(BoxConstraints constraints) => constraints.biggest;
 
-  /// Override this method to lay out and position all children given
-  /// this widget's size and the specified constraints. This method
-  /// must call [layoutChild] for each child. It should also specify
-  /// the final position of each child with [positionChild].
-  void performLayout(Size size, BoxConstraints constraints);
+  /// Override this method to lay out and position all children given this
+  /// widget's size. This method must call [layoutChild] for each child. It
+  /// should also specify the final position of each child with [positionChild].
+  void performLayout(Size size);
 
   /// Override this method to return true when the children need to be
   /// laid out. This should compare the fields of the current delegate
@@ -257,7 +256,7 @@ class RenderCustomMultiChildLayoutBox extends RenderBox
   }
 
   void performLayout() {
-    delegate._callPerformLayout(size, constraints, firstChild);
+    delegate._callPerformLayout(size, firstChild);
   }
 
   void paint(PaintingContext context, Offset offset) {

packages/flutter/lib/src/rendering/debug.dart

@@ -53,9 +53,6 @@ bool debugPaintPointersEnabled = false;
 /// The color to use when reporting pointers.
 int debugPaintPointersColorValue = 0x00BBBB;
 
-/// The color to use when painting [RenderErrorBox] objects in checked mode.
-Color debugErrorBoxColor = const Color(0xFFFF0000);
-
 /// Overlay a rotating set of colors when repainting layers in checked mode.
 bool debugRepaintRainbowEnabled = false;
 

packages/flutter/lib/src/rendering/error.dart

@@ -2,15 +2,56 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui' as ui show Paragraph, ParagraphBuilder, ParagraphStyle, TextStyle;
+
 import 'box.dart';
-import 'debug.dart';
 import 'object.dart';
 
 const double _kMaxWidth = 100000.0;
 const double _kMaxHeight = 100000.0;
 
-/// A render object used as a placeholder when an error occurs
+/// A render object used as a placeholder when an error occurs.
+///
+/// The box will be painted in the color given by the
+/// [RenderErrorBox.backgroundColor] static property.
+///
+/// A message can be provided. To simplify the class and thus help reduce the
+/// likelihood of this class itself being the source of errors, the message
+/// cannot be changed once the object has been created. If provided, the text
+/// will be painted on top of the background, using the styles given by the
+/// [RenderErrorBox.textStyle] and [RenderErrorBox.paragraphStyle] static
+/// properties.
+///
+/// Again to help simplify the class, this box tries to be 100000.0 pixels wide
+/// and high, to approximate being infinitely high but without using infinities.
 class RenderErrorBox extends RenderBox {
+  /// Constructs a RenderErrorBox render object.
+  ///
+  /// A message can optionally be provided. If a message is provided, an attempt
+  /// will be made to render the message when the box paints.
+  RenderErrorBox([ this.message = '' ]) {
+    try {
+      if (message != '') {
+        // This class is intentionally doing things using the low-level
+        // primitives to avoid depending on any subsystems that may have ended
+        // up in an unstable state -- after all, this class is mainly used when
+        // things have gone wrong.
+        //
+        // Generally, the much better way to draw text in a RenderObject is to
+        // use the TextPainter class. If you're looking for code to crib from,
+        // see the paragraph.dart file and the RenderParagraph class.
+        ui.ParagraphBuilder builder = new ui.ParagraphBuilder();
+        builder.pushStyle(textStyle);
+        builder.addText(message);
+        _paragraph = builder.build(paragraphStyle);
+      }
+    } catch (e) { }
+  }
+
+  /// The message to attempt to display at paint time.
+  final String message;
+
+  ui.Paragraph _paragraph;
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     return constraints.constrainWidth(0.0);
@@ -36,8 +77,36 @@ class RenderErrorBox extends RenderBox {
     size = constraints.constrain(const Size(_kMaxWidth, _kMaxHeight));
   }
 
+  /// The color to use when painting the background of [RenderErrorBox] objects.
+  static Color backgroundColor = const Color(0xF0900000);
+
+  /// The text style to use when painting [RenderErrorBox] objects.
+  static ui.TextStyle textStyle = new ui.TextStyle(
+    color: const Color(0xFFFFFF00),
+    fontFamily: 'monospace',
+    fontSize: 7.0
+  );
+
+  /// The paragraph style to use when painting [RenderErrorBox] objects.
+  static ui.ParagraphStyle paragraphStyle = new ui.ParagraphStyle(
+    lineHeight: 0.25 // TODO(ianh): https://github.com/flutter/flutter/issues/2460 will affect this
+  );
+
   void paint(PaintingContext context, Offset offset) {
-    context.canvas.drawRect(offset & size, new Paint() .. color = debugErrorBoxColor);
+    try {
+      context.canvas.drawRect(offset & size, new Paint() .. color = backgroundColor);
+      if (_paragraph != null) {
+        // See the comment in the RenderErrorBox constructor. This is not the
+        // code you want to be copying and pasting. :-)
+        if (parent is RenderBox) {
+          RenderBox parentBox = parent;
+          _paragraph.maxWidth = parentBox.size.width;
+        } else {
+          _paragraph.maxWidth = size.width;
+        }
+        _paragraph.layout();
+        _paragraph.paint(context.canvas, offset);
+      }
+    } catch (e) { }
   }
-
 }

packages/flutter/lib/src/services/asset_bundle.dart

@@ -96,14 +96,11 @@ class MojoAssetBundle extends CachingAssetBundle {
 }
 
 AssetBundle _initRootBundle() {
-  try {
-    AssetBundleProxy bundle = new AssetBundleProxy.fromHandle(
-      new core.MojoHandle(ui.takeRootBundleHandle())
-    );
-    return new MojoAssetBundle(bundle);
-  } catch (e) {
+  int h = ui.takeRootBundleHandle();
+  if (h == core.MojoHandle.INVALID)
     return null;
-  }
+  core.MojoHandle handle = new core.MojoHandle(h);
+  return new MojoAssetBundle(new AssetBundleProxy.fromHandle(handle));
 }
 
 final AssetBundle rootBundle = _initRootBundle();

packages/flutter/lib/src/services/binding.dart

-// Copyright 2015 The Chromium Authors. All rights reserved.
+// Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:ui' as ui;
+import 'package:flutter/shell.dart';
 
-import 'package:mojo/application.dart';
-import 'package:mojo/bindings.dart' as bindings;
-import 'package:mojo/core.dart' as core;
-import 'package:mojo/mojo/service_provider.mojom.dart' as mojom;
-import 'package:mojo/mojo/shell.mojom.dart' as mojom;
+export 'package:flutter/shell.dart';
 
 /// Base class for mixins that provide singleton services (also known as
 /// "bindings").
@@ -41,84 +37,9 @@ abstract class BindingBase {
   String toString() => '<$runtimeType>';
 }
 
-// A replacement for shell.connectToService.  Implementations should return true
-// if they handled the request, or false if the request should fall through
-// to the default requestService.
-typedef bool OverrideConnectToService(String url, Object proxy);
-
-abstract class MojoShell extends BindingBase {
-
+abstract class Services extends BindingBase {
   void initInstances() {
     super.initInstances();
-    _instance = this;
-  }
-
-  static MojoShell _instance;
-  static MojoShell get instance => _instance;
-
-  static mojom.ShellProxy _initShellProxy() {
-    core.MojoHandle shellHandle = new core.MojoHandle(ui.takeShellProxyHandle());
-    if (!shellHandle.isValid)
-      return null;
-    return new mojom.ShellProxy.fromHandle(shellHandle);
-  }
-  final mojom.Shell _shell = _initShellProxy()?.ptr;
-
-  static ApplicationConnection _initEmbedderConnection() {
-    core.MojoHandle servicesHandle = new core.MojoHandle(ui.takeServicesProvidedByEmbedder());
-    core.MojoHandle exposedServicesHandle = new core.MojoHandle(ui.takeServicesProvidedToEmbedder());
-    if (!servicesHandle.isValid || !exposedServicesHandle.isValid)
-      return null;
-    mojom.ServiceProviderProxy services = new mojom.ServiceProviderProxy.fromHandle(servicesHandle);
-    mojom.ServiceProviderStub exposedServices = new mojom.ServiceProviderStub.fromHandle(exposedServicesHandle);
-    return new ApplicationConnection(exposedServices, services);
-  }
-  final ApplicationConnection _embedderConnection = _initEmbedderConnection();
-
-  /// Attempts to connect to an application via the Mojo shell.
-  ApplicationConnection connectToApplication(String url) {
-    if (_shell == null)
-      return null;
-    mojom.ServiceProviderProxy services = new mojom.ServiceProviderProxy.unbound();
-    mojom.ServiceProviderStub exposedServices = new mojom.ServiceProviderStub.unbound();
-    _shell.connectToApplication(url, services, exposedServices);
-    return new ApplicationConnection(exposedServices, services);
-  }
-
-  /// Set this to intercept calls to [connectToService()] and supply an
-  /// alternative implementation of a service (for example, a mock for testing).
-  OverrideConnectToService overrideConnectToService;
-
-  /// Attempts to connect to a service implementing the interface for the given proxy.
-  /// If an application URL is specified, the service will be requested from that application.
-  /// Otherwise, it will be requested from the embedder (the Flutter engine).
-  void connectToService(String url, bindings.ProxyBase proxy) {
-    if (overrideConnectToService != null && overrideConnectToService(url, proxy))
-      return;
-    if (url == null || _shell == null) {
-      // If the application URL is null, it means the service to connect
-      // to is one provided by the embedder.
-      // If the applircation URL isn't null but there's no shell, then
-      // ask the embedder in case it provides it. (For example, if you're
-      // running on Android without the Mojo shell, then you can obtain
-      // the media service from the embedder directly, instead of having
-      // to ask the media application for it.)
-      // This makes it easier to write an application that works both
-      // with and without a Mojo environment.
-      _embedderConnection?.requestService(proxy);
-      return;
-    }
-    mojom.ServiceProviderProxy services = new mojom.ServiceProviderProxy.unbound();
-    _shell.connectToApplication(url, services, null);
-    core.MojoMessagePipe pipe = new core.MojoMessagePipe();
-    proxy.impl.bind(pipe.endpoints[0]);
-    services.ptr.connectToService(proxy.serviceName, pipe.endpoints[1]);
-    services.close();
-  }
-
-  /// Registers a service to expose to the embedder.
-  void provideService(String interfaceName, ServiceFactory factory) {
-    _embedderConnection?.provideService(interfaceName, factory);
+    new MojoShell();
   }
 }
-MojoShell get shell => MojoShell.instance;

packages/flutter/lib/src/services/print.dart

@@ -46,9 +46,5 @@ void _debugPrintTask() {
 }
 
 void debugPrintStack() {
-  try {
-    throw new Exception();
-  } catch (e, stack) {
-    debugPrint(stack.toString());
-  }
+  debugPrint(StackTrace.current.toString());
 }

packages/flutter/lib/src/widgets/app.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:ui' as ui show Locale, WindowPadding, window;
+
+import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
+
+import 'asset_vendor.dart';
+import 'basic.dart';
+import 'binding.dart';
+import 'checked_mode_banner.dart';
+import 'framework.dart';
+import 'locale_query.dart';
+import 'media_query.dart';
+import 'navigator.dart';
+import 'performance_overlay.dart';
+import 'semantics_debugger.dart';
+import 'title.dart';
+
+AssetBundle _initDefaultBundle() {
+  if (rootBundle != null)
+    return rootBundle;
+  return new NetworkAssetBundle(Uri.base);
+}
+
+final AssetBundle _defaultBundle = _initDefaultBundle();
+
+class RouteArguments {
+  const RouteArguments({ this.context });
+  final BuildContext context;
+}
+typedef Widget RouteBuilder(RouteArguments args);
+
+typedef Future<LocaleQueryData> LocaleChangedCallback(Locale locale);
+
+class WidgetsApp extends StatefulComponent {
+  WidgetsApp({
+    Key key,
+    this.title,
+    this.textStyle,
+    this.color,
+    this.routes: const <String, RouteBuilder>{},
+    this.onGenerateRoute,
+    this.onLocaleChanged,
+    this.showPerformanceOverlay: false,
+    this.showSemanticsDebugger: false,
+    this.debugShowCheckedModeBanner: true
+  }) : super(key: key) {
+    assert(routes != null);
+    assert(routes.containsKey(Navigator.defaultRouteName) || onGenerateRoute != null);
+    assert(showPerformanceOverlay != null);
+    assert(showSemanticsDebugger != null);
+  }
+
+  /// A one-line description of this app for use in the window manager.
+  final String title;
+
+  /// The default text style for [Text] in the application.
+  final TextStyle textStyle;
+
+  /// The primary color to use for the application in the operating system
+  /// interface.
+  ///
+  /// For example, on Android this is the color used for the application in the
+  /// application switcher.
+  final Color color;
+
+  /// The default table of routes for the application. When the
+  /// [Navigator] is given a named route, the name will be looked up
+  /// in this table first. If the name is not available, then
+  /// [onGenerateRoute] will be called instead.
+  final Map<String, RouteBuilder> routes;
+
+  /// The route generator callback used when the app is navigated to a
+  /// named route but the name is not in the [routes] table.
+  final RouteFactory onGenerateRoute;
+
+  /// Callback that is invoked when the operating system changes the
+  /// current locale.
+  final LocaleChangedCallback onLocaleChanged;
+
+  /// Turns on a performance overlay.
+  /// https://flutter.io/debugging/#performanceoverlay
+  final bool showPerformanceOverlay;
+
+  /// Turns on an overlay that shows the accessibility information
+  /// reported by the framework.
+  final bool showSemanticsDebugger;
+
+  /// Turns on a "SLOW MODE" little banner in checked mode to indicate
+  /// that the app is in checked mode. This is on by default (in
+  /// checked mode), to turn it off, set the constructor argument to
+  /// false. In release mode this has no effect.
+  ///
+  /// To get this banner in your application if you're not using
+  /// WidgetsApp, include a [CheckedModeBanner] widget in your app.
+  ///
+  /// This banner is intended to avoid people complaining that your
+  /// app is slow when it's in checked mode. In checked mode, Flutter
+  /// enables a large number of expensive diagnostics to aid in
+  /// development, and so performance in checked mode is not
+  /// representative of what will happen in release mode.
+  final bool debugShowCheckedModeBanner;
+
+  WidgetsAppState<WidgetsApp> createState() => new WidgetsAppState<WidgetsApp>();
+}
+
+EdgeDims _getPadding(ui.WindowPadding padding) {
+  return new EdgeDims.TRBL(padding.top, padding.right, padding.bottom, padding.left);
+}
+
+class WidgetsAppState<T extends WidgetsApp> extends State<T> implements BindingObserver {
+
+  GlobalObjectKey _navigator;
+
+  LocaleQueryData _localeData;
+
+  void initState() {
+    super.initState();
+    _navigator = new GlobalObjectKey(this);
+    didChangeLocale(ui.window.locale);
+    WidgetFlutterBinding.instance.addObserver(this);
+  }
+
+  void dispose() {
+    WidgetFlutterBinding.instance.removeObserver(this);
+    super.dispose();
+  }
+
+  bool didPopRoute() {
+    assert(mounted);
+    NavigatorState navigator = _navigator.currentState;
+    assert(navigator != null);
+    bool result = false;
+    navigator.openTransaction((NavigatorTransaction transaction) {
+      result = transaction.pop();
+    });
+    return result;
+  }
+
+  void didChangeMetrics() {
+    setState(() {
+      // The properties of ui.window have changed. We use them in our build
+      // function, so we need setState(), but we don't cache anything locally.
+    });
+  }
+
+  void didChangeLocale(Locale locale) {
+    if (config.onLocaleChanged != null) {
+      config.onLocaleChanged(locale).then((LocaleQueryData data) {
+        if (mounted)
+          setState(() { _localeData = data; });
+      });
+    }
+  }
+
+  void didChangeAppLifecycleState(AppLifecycleState state) { }
+
+  NavigatorObserver get navigatorObserver => null;
+
+  Widget build(BuildContext context) {
+    if (config.onLocaleChanged != null && _localeData == null) {
+      // If the app expects a locale but we don't yet know the locale, then
+      // don't build the widgets now.
+      // TODO(ianh): Make this unnecessary. See https://github.com/flutter/flutter/issues/1865
+      return new Container();
+    }
+
+    Widget result = new MediaQuery(
+      data: new MediaQueryData(
+        size: ui.window.size,
+        devicePixelRatio: ui.window.devicePixelRatio,
+        padding: _getPadding(ui.window.padding)
+      ),
+      child: new LocaleQuery(
+        data: _localeData,
+        child: new DefaultTextStyle(
+          style: config.textStyle,
+          child: new AssetVendor(
+            bundle: _defaultBundle,
+            devicePixelRatio: ui.window.devicePixelRatio,
+            child: new Title(
+              title: config.title,
+              color: config.color,
+              child: new Navigator(
+                key: _navigator,
+                initialRoute: ui.window.defaultRouteName,
+                onGenerateRoute: config.onGenerateRoute,
+                observer: navigatorObserver
+              )
+            )
+          )
+        )
+      )
+    );
+    if (config.showPerformanceOverlay) {
+      result = new Stack(
+        children: <Widget>[
+          result,
+          new Positioned(bottom: 0.0, left: 0.0, right: 0.0, child: new PerformanceOverlay.allEnabled()),
+        ]
+      );
+    }
+    if (config.showSemanticsDebugger) {
+      result = new SemanticsDebugger(
+        child: result
+      );
+    }
+    assert(() {
+      if (config.debugShowCheckedModeBanner) {
+        result = new CheckedModeBanner(
+          child: result
+        );
+      }
+      return true;
+    });
+    return result;
+  }
+
+}

packages/flutter/lib/src/widgets/basic.dart

@@ -999,6 +999,7 @@ class BlockBody extends MultiChildRenderObjectWidget {
   }
 }
 
+/// A base class for widgets that accept [Positioned] children.
 abstract class StackRenderObjectWidgetBase extends MultiChildRenderObjectWidget {
   StackRenderObjectWidgetBase({
     List<Widget> children: _emptyWidgetList,

packages/flutter/lib/src/widgets/binding.dart

@@ -24,7 +24,7 @@ class BindingObserver {
 
 /// A concrete binding for applications based on the Widgets framework.
 /// This is the glue that binds the framework to the Flutter engine.
-class WidgetFlutterBinding extends BindingBase with Scheduler, Gesturer, MojoShell, Renderer {
+class WidgetFlutterBinding extends BindingBase with Scheduler, Gesturer, Services, Renderer {
 
   /// Creates and initializes the WidgetFlutterBinding. This constructor is
   /// idempotent; calling it a second time will just return the

packages/flutter/lib/src/widgets/dismissable.dart

@@ -256,7 +256,7 @@ class _DismissableState extends State<Dismissable> {
           assert(_resizeAnimation.status == AnimationStatus.completed);
           throw new WidgetError(
             'Dismissable widget completed its resize animation without being removed from the tree.\n'
-            'Make sure to implement the onDismissed handler and to immediately remove the Dismissable\n'
+            'Make sure to implement the onDismissed handler and to immediately remove the Dismissable '
             'widget from the application once that handler has fired.'
           );
         }

packages/flutter/lib/src/widgets/drag_target.dart

@@ -234,7 +234,7 @@ class _DraggableState<T> extends State<DraggableBase<T>> {
       _activeCount += 1;
     });
     return new _DragAvatar<T>(
-      overlay: Overlay.of(context),
+      overlay: Overlay.of(context, debugRequiredFor: config),
       data: config.data,
       initialPosition: position,
       dragStartPoint: dragStartPoint,
@@ -249,6 +249,7 @@ class _DraggableState<T> extends State<DraggableBase<T>> {
   }
 
   Widget build(BuildContext context) {
+    assert(Overlay.of(context, debugRequiredFor: config) != null);
     final bool canDrag = config.maxSimultaneousDrags == null ||
                          _activeCount < config.maxSimultaneousDrags;
     final bool showChild = _activeCount == 0 || config.childWhenDragging == null;

packages/flutter/lib/src/widgets/framework.dart

@@ -149,8 +149,12 @@ abstract class GlobalKey<T extends State<StatefulComponent>> extends Key {
       message += 'The following GlobalKey was found multiple times among mounted elements: $key (${_debugDuplicates[key]} instances)\n';
       message += 'The most recently registered instance is: ${_registry[key]}\n';
     }
-    if (!_debugDuplicates.isEmpty)
-      throw new WidgetError('Incorrect GlobalKey usage.', message);
+    if (!_debugDuplicates.isEmpty) {
+      throw new WidgetError(
+        'Incorrect GlobalKey usage.\n'
+        '$message'
+      );
+    }
     return true;
   }
 
@@ -1012,8 +1016,24 @@ abstract class Element<T extends Widget> implements BuildContext {
   }
 }
 
+/// A widget that renders an exception's message. This widget is used when a
+/// build function fails, to help with determining where the problem lies.
+/// Exceptions are also logged to the console, which you can read using `flutter
+/// logs`. The console will also include additional information such as the
+/// stack trace for the exception.
 class ErrorWidget extends LeafRenderObjectWidget {
-  RenderBox createRenderObject() => new RenderErrorBox();
+  ErrorWidget(
+    Object exception
+  ) : message = _stringify(exception),
+      super(key: new UniqueKey());
+  final String message;
+  static String _stringify(Object exception) {
+    try {
+      return exception.toString();
+    } catch (e) { }
+    return 'Error';
+  }
+  RenderBox createRenderObject() => new RenderErrorBox(message);
 }
 
 typedef void BuildScheduler(BuildableElement element);
@@ -1109,9 +1129,10 @@ abstract class BuildableElement<T extends Widget> extends Element<T> {
       }
       if (_debugStateLocked && (!_debugAllowIgnoredCallsToMarkNeedsBuild || !dirty)) {
         throw new WidgetError(
-          'Cannot mark this component as needing to build because the framework is '
-          'already in the process of building widgets. A widget can be marked as '
-          'needing to be built during the build phase only if one if its ancestor '
+          'setState() or markNeedsBuild() called during build.\n'
+          'This component cannot be marked as needing to build because the framework '
+          'is already in the process of building widgets. A widget can be marked as '
+          'needing to be built during the build phase only if one if its ancestors '
           'is currently building. This exception is allowed because the framework '
           'builds parent widgets before children, which means a dirty descendant '
           'will always be built. Otherwise, the framework might not visit this '
@@ -1208,15 +1229,18 @@ abstract class ComponentElement<T extends Widget> extends BuildableElement<T> {
       assert(() {
         if (built == null) {
           throw new WidgetError(
-            'A build function returned null. Build functions must never return null.',
-            'The offending widget is: $widget'
+            'A build function returned null.\n'
+            'The offending widget is: $widget\n'
+            'Build functions must never return null. '
+            'To return an empty space that causes the building widget to fill available room, return "new Container()". '
+            'To return an empty space that takes as little room as possible, return "new Container(width: 0.0, height: 0.0)".'
           );
         }
         return true;
       });
     } catch (e, stack) {
       _debugReportException('building $_widget', e, stack);
-      built = new ErrorWidget();
+      built = new ErrorWidget(e);
     } finally {
       // We delay marking the element as clean until after calling _builder so
       // that attempts to markNeedsBuild() during build() will be ignored.
@@ -1228,7 +1252,7 @@ abstract class ComponentElement<T extends Widget> extends BuildableElement<T> {
       assert(_child != null);
     } catch (e, stack) {
       _debugReportException('building $_widget', e, stack);
-      built = new ErrorWidget();
+      built = new ErrorWidget(e);
       _child = updateChild(null, built, slot);
     }
   }
@@ -1289,7 +1313,11 @@ class StatefulComponentElement<T extends StatefulComponent, U extends State<T>>
     assert(() {
       if (_state._debugLifecycleState == _StateLifecycle.initialized)
         return true;
-      throw new WidgetError('${_state.runtimeType}.initState failed to call super.initState.');
+      throw new WidgetError(
+        '${_state.runtimeType}.initState failed to call super.initState.\n'
+        'initState() implementations must always call their superclass initState() method, to ensure '
+        'that the entire widget is initialized correctly.'
+      );
     });
     assert(() { _state._debugLifecycleState = _StateLifecycle.ready; return true; });
     super._firstBuild();
@@ -1324,7 +1352,11 @@ class StatefulComponentElement<T extends StatefulComponent, U extends State<T>>
     assert(() {
       if (_state._debugLifecycleState == _StateLifecycle.defunct)
         return true;
-      throw new WidgetError('${_state.runtimeType}.dispose failed to call super.dispose.');
+      throw new WidgetError(
+        '${_state.runtimeType}.dispose failed to call super.dispose.\n'
+        'dispose() implementations must always call their superclass dispose() method, to ensure '
+        'that all the resources used by the widget are fully released.'
+      );
     });
     assert(!dirty); // See BuildableElement.unmount for why this is important.
     _state._element = null;
@@ -1381,12 +1413,15 @@ class ParentDataElement extends _ProxyElement<ParentDataWidget> {
       }
       if (ancestor != null && badAncestors.isEmpty)
         return true;
-      throw new WidgetError('Incorrect use of ParentDataWidget.', widget.debugDescribeInvalidAncestorChain(
-        description: "$this",
-        ownershipChain: parent.debugGetOwnershipChain(10),
-        foundValidAncestor: ancestor != null,
-        badAncestors: badAncestors
-      ));
+      throw new WidgetError(
+        'Incorrect use of ParentDataWidget.\n' +
+        widget.debugDescribeInvalidAncestorChain(
+          description: "$this",
+          ownershipChain: parent.debugGetOwnershipChain(10),
+          foundValidAncestor: ancestor != null,
+          badAncestors: badAncestors
+        )
+      );
     });
     super.mount(parent, slot);
   }
@@ -1507,7 +1542,14 @@ abstract class RenderObjectElement<T extends RenderObjectWidget> extends Buildab
     // 'BuildContext' argument which you can pass to Theme.of() and other
     // InheritedWidget APIs which eventually trigger a rebuild.)
     assert(() {
-      throw new WidgetError('$runtimeType failed to implement reinvokeBuilders(), but got marked dirty.');
+      throw new WidgetError(
+        '$runtimeType failed to implement reinvokeBuilders(), but got marked dirty.\n'
+        'If a RenderObjectElement subclass supports being marked dirty, then the '
+        'reinvokeBuilders() method must be implemented.\n'
+        'If a RenderObjectElement uses a builder callback, it must support being '
+        'marked dirty, because builder callbacks can register the object as having '
+        'an Inherited dependency.'
+      );
     });
   }
 
@@ -1812,7 +1854,11 @@ class MultiChildRenderObjectElement<T extends MultiChildRenderObjectWidget> exte
         continue; // when these nodes are reordered, we just reassign the data
 
       if (!idSet.add(child.key)) {
-        throw new WidgetError('If multiple keyed nodes exist as children of another node, they must have unique keys. $widget has multiple children with key "${child.key}".');
+        throw new WidgetError(
+          'Duplicate keys found.\n'
+          'If multiple keyed nodes exist as children of another node, they must have unique keys.\n'
+          '$widget has multiple children with key "${child.key}".'
+        );
       }
     }
     return false;
@@ -1841,16 +1887,10 @@ class MultiChildRenderObjectElement<T extends MultiChildRenderObjectWidget> exte
   }
 }
 
-class WidgetError extends Error {
-  WidgetError(String message, [ String rawDetails = '' ]) {
-    rawDetails = rawDetails.trimRight(); // remove trailing newlines
-    if (rawDetails != '')
-      _message = '$message\n$rawDetails';
-    else
-      _message = message;
-  }
-  String _message;
-  String toString() => _message;
+class WidgetError extends AssertionError {
+  WidgetError(this.message);
+  final String message;
+  String toString() => message;
 }
 
 typedef void WidgetsExceptionHandler(String context, dynamic exception, StackTrace stack);

packages/flutter/lib/src/widgets/gesture_detector.dart

@@ -71,11 +71,20 @@ class GestureDetector extends StatelessComponent {
       bool havePan = onPanStart != null || onPanUpdate != null || onPanEnd != null;
       bool haveScale = onScaleStart != null || onScaleUpdate != null || onScaleEnd != null;
       if (havePan || haveScale) {
-        if (havePan && haveScale)
-          throw new WidgetError('Having both a pan gesture recognizer and a scale gesture recognizer is redundant; scale is a superset of pan. Just use the scale gesture recognizer.');
+        if (havePan && haveScale) {
+          throw new WidgetError(
+            'Incorrect GestureDetector arguments.\n'
+            'Having both a pan gesture recognizer and a scale gesture recognizer is redundant; scale is a superset of pan. Just use the scale gesture recognizer.'
+          );
+        }
         String recognizer = havePan ? 'pan' : 'scale';
-        if (haveVerticalDrag && haveHorizontalDrag)
-          throw new WidgetError('Simultaneously having a vertical drag gesture recognizer, a horizontal drag gesture recognizer, and a $recognizer gesture recognizer will result in the $recognizer gesture recognizer being ignored, since the other two will catch all drags.');
+        if (haveVerticalDrag && haveHorizontalDrag) {
+          throw new WidgetError(
+            'Incorrect GestureDetector arguments.\n'
+            'Simultaneously having a vertical drag gesture recognizer, a horizontal drag gesture recognizer, and a $recognizer gesture recognizer '
+            'will result in the $recognizer gesture recognizer being ignored, since the other two will catch all drags.'
+          );
+        }
       }
       return true;
     });
@@ -279,8 +288,15 @@ class RawGestureDetectorState extends State<RawGestureDetector> {
   /// the gesture detector should be enabled.
   void replaceGestureRecognizers(Map<Type, GestureRecognizerFactory> gestures) {
     assert(() {
-      if (!RenderObject.debugDoingLayout)
-        throw new WidgetError('replaceGestureRecognizers() can only be called during the layout phase.');
+      if (!RenderObject.debugDoingLayout) {
+        throw new WidgetError(
+          'Unexpected call to replaceGestureRecognizers() method of RawGestureDetectorState.\n'
+          'The replaceGestureRecognizers() method can only be called during the layout phase. '
+          'To set the gesture recognisers at other times, trigger a new build using setState() '
+          'and provide the new gesture recognisers as constructor arguments to the corresponding '
+          'RawGestureDetector or GestureDetector object.'
+        );
+      }
       return true;
     });
     _syncAll(gestures);

packages/flutter/lib/src/widgets/heroes.dart

@@ -113,8 +113,8 @@ class Hero extends StatefulComponent {
           if (tagHeroes.containsKey(key)) {
             new WidgetError(
               'There are multiple heroes that share the same key within the same subtree.\n'
-              'Within each subtree for which heroes are to be animated (typically a PageRoute subtree),\n'
-              'either each Hero must have a unique tag, or, all the heroes with a particular tag must\n'
+              'Within each subtree for which heroes are to be animated (typically a PageRoute subtree), '
+              'either each Hero must have a unique tag, or, all the heroes with a particular tag must '
               'have different keys.\n'
               'In this case, the tag "$tag" had multiple heroes with the key "$key".'
             );

packages/flutter/lib/src/widgets/mimic.dart

@@ -137,6 +137,9 @@ class Mimic extends StatelessComponent {
 }
 
 /// A widget that can be copied by a [Mimic].
+///
+/// This widget's State, [MimicableState], contains an API for initiating the
+/// mimic operation.
 class Mimicable extends StatefulComponent {
   Mimicable({ Key key, this.child }) : super(key: key);
 
@@ -173,7 +176,18 @@ class MimicableState extends State<Mimicable> {
   /// passing it to a [Mimic] widget. To mimic the child in the
   /// [Overlay], consider using [liftToOverlay()] instead.
   MimicableHandle startMimic() {
-    assert(_placeholderSize == null);
+    assert(() {
+      if (_placeholderSize != null) {
+        throw new WidgetError(
+          'Mimicable started while already active.\n'
+          'When startMimic() or liftToOverlay() is called on a MimicableState, the mimic becomes active. '
+          'While active, it cannot be reactivated until it is stopped. '
+          'To stop a Mimicable started with startMimic(), call the MimicableHandle object\'s stopMimic() method. '
+          'To stop a Mimicable started with liftToOverlay(), call dispose() on the MimicOverlayEntry.'
+        );
+      }
+      return true;
+    });
     RenderBox box = context.findRenderObject();
     assert(box != null);
     assert(box.hasSize);
@@ -193,8 +207,7 @@ class MimicableState extends State<Mimicable> {
   /// had when the mimicking process started and (2) the child will be
   /// placed in the enclosing overlay.
   MimicOverlayEntry liftToOverlay() {
-    OverlayState overlay = Overlay.of(context);
-    assert(overlay != null); // You need an overlay to lift into.
+    OverlayState overlay = Overlay.of(context, debugRequiredFor: config);
     MimicOverlayEntry entry = new MimicOverlayEntry._(startMimic());
     overlay.insert(entry._overlayEntry);
     return entry;

packages/flutter/lib/src/widgets/navigator.dart

@@ -260,8 +260,12 @@ class Navigator extends StatefulComponent {
   static void openTransaction(BuildContext context, NavigatorTransactionCallback callback) {
     NavigatorState navigator = context.ancestorStateOfType(const TypeMatcher<NavigatorState>());
     assert(() {
-      if (navigator == null)
-        throw new WidgetError('openTransaction called with a context that does not include a Navigator. The context passed to the Navigator.openTransaction() method must be that of a widget that is a descendant of a Navigator widget.');
+      if (navigator == null) {
+        throw new WidgetError(
+          'openTransaction called with a context that does not include a Navigator.\n'
+          'The context passed to the Navigator.openTransaction() method must be that of a widget that is a descendant of a Navigator widget.'
+        );
+      }
       return true;
     });
     navigator.openTransaction(callback);

packages/flutter/lib/src/widgets/overlay.dart

@@ -71,7 +71,32 @@ class Overlay extends StatefulComponent {
   final List<OverlayEntry> initialEntries;
 
   /// The state from the closest instance of this class that encloses the given context.
-  static OverlayState of(BuildContext context) => context.ancestorStateOfType(const TypeMatcher<OverlayState>());
+  ///
+  /// In checked mode, if the [debugRequiredFor] argument is provided then this
+  /// function will assert that an overlay was found and will throw an exception
+  /// if not. The exception attempts to explain that the calling [Widget] (the
+  /// one given by the [debugRequiredFor] argument) needs an [Overlay] to be
+  /// present to function.
+  static OverlayState of(BuildContext context, { Widget debugRequiredFor }) {
+    OverlayState result = context.ancestorStateOfType(const TypeMatcher<OverlayState>());
+    assert(() {
+      if (debugRequiredFor != null && result == null) {
+        String additional = context.widget != debugRequiredFor
+          ? '\nThe context from which that widget was searching for an overlay was:\n  $context'
+          : '';
+        throw new WidgetError(
+          'No Overlay widget found.\n'
+          '${debugRequiredFor.runtimeType} widgets require an Overlay widget ancestor for correct operation.\n'
+          'The most common way to add an Overlay to an application is to include a MaterialApp or Navigator widget in the runApp() call.\n'
+          'The specific widget that failed to find an overlay was:\n'
+          '  $debugRequiredFor'
+          '$additional'
+        );
+      }
+      return true;
+    });
+    return result;
+  }
 
   OverlayState createState() => new OverlayState();
 }

packages/flutter/lib/src/widgets/pageable_list.dart

@@ -18,6 +18,10 @@ enum ItemsSnapAlignment {
   adjacentItem
 }
 
+/// Scrollable widget that scrolls one "page" at a time.
+///
+/// In a pageable list, one child is visible at a time. Scrolling the list
+/// reveals either the next or previous child.
 class PageableList extends Scrollable {
   PageableList({
     Key key,
@@ -46,17 +50,34 @@ class PageableList extends Scrollable {
     snapOffsetCallback: snapOffsetCallback
   );
 
+  /// Whether the first item should be revealed after scrolling past the last item.
   final bool itemsWrap;
+
+  /// Controls whether a fling always reveals the adjacent item or whether flings can traverse many items.
   final ItemsSnapAlignment itemsSnapAlignment;
+
+  /// Called when the currently visible page changes.
   final ValueChanged<int> onPageChanged;
+
+  /// Used to paint the scrollbar for this list.
   final ScrollableListPainter scrollableListPainter;
+
+  /// The duration used when animating to a given page.
   final Duration duration;
+
+  /// The animation curve to use when animating to a given page.
   final Curve curve;
+
+  /// The list of pages themselves.
   final Iterable<Widget> children;
 
   PageableListState createState() => new PageableListState();
 }
 
+/// State for a [PageableList] widget.
+///
+/// Widgets that subclass [PageableList] can subclass this class to have
+/// sensible default behaviors for pageable lists.
 class PageableListState<T extends PageableList> extends ScrollableState<T> {
   int get _itemCount => config.children?.length ?? 0;
   int _previousItemCount;

packages/flutter/lib/src/widgets/placeholder.dart

@@ -12,6 +12,9 @@ class Placeholder extends StatefulComponent {
   PlaceholderState createState() => new PlaceholderState();
 }
 
+/// State for a [Placeholder] widget.
+///
+/// Useful for setting the child currently displayed by this placeholder widget.
 class PlaceholderState extends State<Placeholder> {
   /// The child that this widget builds.
   ///

packages/flutter/lib/src/widgets/scrollable.dart

@@ -327,15 +327,9 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
     });
     PageStorage.of(context)?.writeState(context, _scrollOffset);
     new ScrollNotification(this, _scrollOffset).dispatch(context);
-    final needsScrollStart = !_isBetweenOnScrollStartAndOnScrollEnd;
-    if (needsScrollStart) {
-      dispatchOnScrollStart();
-      assert(_isBetweenOnScrollStartAndOnScrollEnd);
-    }
+    _startScroll();
     dispatchOnScroll();
-    assert(_isBetweenOnScrollStartAndOnScrollEnd);
-    if (needsScrollStart)
-      dispatchOnScrollEnd();
+    _endScroll();
   }
 
   /// Scroll this widget by the given scroll delta.
@@ -372,8 +366,8 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   Future _animateTo(double newScrollOffset, Duration duration, Curve curve) {
     _controller.stop();
     _controller.value = scrollOffset;
-    _dispatchOnScrollStartIfNeeded();
-    return _controller.animateTo(newScrollOffset, duration: duration, curve: curve).then(_dispatchOnScrollEndIfNeeded);
+    _startScroll();
+    return _controller.animateTo(newScrollOffset, duration: duration, curve: curve).then(_endScroll);
   }
 
   /// Fling the scroll offset with the given velocity.
@@ -401,8 +395,8 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
     Simulation simulation = _createSnapSimulation(scrollVelocity) ?? _createFlingSimulation(scrollVelocity);
     if (simulation == null)
       return new Future.value();
-    _dispatchOnScrollStartIfNeeded();
-    return _controller.animateWith(simulation).then(_dispatchOnScrollEndIfNeeded);
+    _startScroll();
+    return _controller.animateWith(simulation).then(_endScroll);
   }
 
   /// Whether this scrollable should attempt to snap scroll offsets.
@@ -453,14 +447,21 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
     return simulation;
   }
 
-
-  bool _isBetweenOnScrollStartAndOnScrollEnd = false;
+  // When we start an scroll animation, we stop any previous scroll animation.
+  // However, the code that would deliver the onScrollEnd callback is watching
+  // for animations to end using a Future that resolves at the end of the
+  // microtask. That causes animations to "overlap" between the time we start a
+  // new animation and the end of the microtask. By the time the microtask is
+  // over and we check whether to deliver an onScrollEnd callback, we will have
+  // started the new animation (having skipped the onScrollStart) and therefore
+  // we won't deliver the onScrollEnd until the second animation is finished.
+  int _numberOfInProgressScrolls = 0;
 
   /// Calls the onScroll callback.
   ///
   /// Subclasses can override this function to hook the scroll callback.
   void dispatchOnScroll() {
-    assert(_isBetweenOnScrollStartAndOnScrollEnd);
+    assert(_numberOfInProgressScrolls > 0);
     if (config.onScroll != null)
       config.onScroll(_scrollOffset);
   }
@@ -470,11 +471,12 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   }
 
   void _handleDragStart(_) {
-    _dispatchOnScrollStartIfNeeded();
+    _startScroll();
   }
 
-  void _dispatchOnScrollStartIfNeeded() {
-    if (!_isBetweenOnScrollStartAndOnScrollEnd)
+  void _startScroll() {
+    _numberOfInProgressScrolls += 1;
+    if (_numberOfInProgressScrolls == 1)
       dispatchOnScrollStart();
   }
 
@@ -482,8 +484,7 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   ///
   /// Subclasses can override this function to hook the scroll start callback.
   void dispatchOnScrollStart() {
-    assert(!_isBetweenOnScrollStartAndOnScrollEnd);
-    _isBetweenOnScrollStartAndOnScrollEnd = true;
+    assert(_numberOfInProgressScrolls == 1);
     if (config.onScrollStart != null)
       config.onScrollStart(_scrollOffset);
   }
@@ -495,11 +496,12 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   Future _handleDragEnd(Velocity velocity) {
     double scrollVelocity = pixelDeltaToScrollOffset(velocity.pixelsPerSecond) / Duration.MILLISECONDS_PER_SECOND;
     // The gesture velocity properties are pixels/second, config min,max limits are pixels/ms
-    return fling(scrollVelocity.clamp(-kMaxFlingVelocity, kMaxFlingVelocity)).then(_dispatchOnScrollEndIfNeeded);
+    return fling(scrollVelocity.clamp(-kMaxFlingVelocity, kMaxFlingVelocity)).then(_endScroll);
   }
 
-  void _dispatchOnScrollEndIfNeeded(_) {
-    if (_isBetweenOnScrollStartAndOnScrollEnd)
+  void _endScroll([_]) {
+    _numberOfInProgressScrolls -= 1;
+    if (_numberOfInProgressScrolls == 0)
       dispatchOnScrollEnd();
   }
 
@@ -507,8 +509,7 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   ///
   /// Subclasses can override this function to hook the scroll end callback.
   void dispatchOnScrollEnd() {
-    assert(_isBetweenOnScrollStartAndOnScrollEnd);
-    _isBetweenOnScrollStartAndOnScrollEnd = false;
+    assert(_numberOfInProgressScrolls == 0);
     if (config.onScrollEnd != null)
       config.onScrollEnd(_scrollOffset);
   }

packages/flutter/lib/widgets.dart

@@ -5,6 +5,7 @@
 /// The Flutter widget framework.
 library widgets;
 
+export 'src/widgets/app.dart';
 export 'src/widgets/asset_vendor.dart';
 export 'src/widgets/auto_layout.dart';
 export 'src/widgets/basic.dart';

packages/flutter/test/rendering/rendering_tester.dart

@@ -26,7 +26,7 @@ enum EnginePhase {
   composite
 }
 
-class TestRenderingFlutterBinding extends BindingBase with Scheduler, MojoShell, Renderer, Gesturer {
+class TestRenderingFlutterBinding extends BindingBase with Scheduler, Services, Renderer, Gesturer {
   void initRenderView() {
     if (renderView == null) {
       renderView = new TestRenderView();
@@ -58,7 +58,7 @@ void layout(RenderBox box, { BoxConstraints constraints, EnginePhase phase: Engi
 
   _renderer ??= new TestRenderingFlutterBinding();
 
-  renderer.renderView.child = null; 
+  renderer.renderView.child = null;
   if (constraints != null) {
     box = new RenderPositionedBox(
       child: new RenderConstrainedBox(

packages/flutter/test/widget/custom_multi_child_layout_test.dart

@@ -17,16 +17,15 @@ class TestMultiChildLayoutDelegate extends MultiChildLayoutDelegate {
   }
 
   Size performLayoutSize;
-  BoxConstraints performLayoutConstraints;
   Size performLayoutSize0;
   Size performLayoutSize1;
   bool performLayoutIsChild;
 
-  void performLayout(Size size, BoxConstraints constraints) {
+  void performLayout(Size size) {
     assert(!RenderObject.debugCheckingIntrinsics);
     expect(() {
       performLayoutSize = size;
-      performLayoutConstraints = constraints;
+      BoxConstraints constraints = new BoxConstraints.loose(size);
       performLayoutSize0 = layoutChild(0, constraints);
       performLayoutSize1 = layoutChild(1, constraints);
       performLayoutIsChild = isChild('fred');
@@ -68,10 +67,6 @@ void main() {
 
       expect(delegate.performLayoutSize.width, 200.0);
       expect(delegate.performLayoutSize.height, 300.0);
-      expect(delegate.performLayoutConstraints.minWidth, 0.0);
-      expect(delegate.performLayoutConstraints.maxWidth, 800.0);
-      expect(delegate.performLayoutConstraints.minHeight, 0.0);
-      expect(delegate.performLayoutConstraints.maxHeight, 600.0);
       expect(delegate.performLayoutSize0.width, 150.0);
       expect(delegate.performLayoutSize0.height, 100.0);
       expect(delegate.performLayoutSize1.width, 100.0);

packages/flutter/test/widget/scroll_events_test.dart

@@ -95,4 +95,45 @@ void main() {
       expect(log, equals(['scrollstart', 'scroll', 'scroll', 'scrollend']));
     });
   });
+
+  test('Scroll during animation', () {
+    testWidgets((WidgetTester tester) {
+      GlobalKey<ScrollableState> scrollKey = new GlobalKey<ScrollableState>();
+      List<String> log = <String>[];
+      tester.pumpWidget(_buildScroller(key: scrollKey, log: log));
+
+      expect(log, equals([]));
+      scrollKey.currentState.scrollTo(100.0, duration: const Duration(seconds: 1));
+      expect(log, equals(['scrollstart']));
+      tester.pump(const Duration(milliseconds: 100));
+      expect(log, equals(['scrollstart']));
+      tester.pump(const Duration(milliseconds: 100));
+      expect(log, equals(['scrollstart', 'scroll']));
+      scrollKey.currentState.scrollTo(100.0, duration: const Duration(seconds: 1));
+      expect(log, equals(['scrollstart', 'scroll']));
+      tester.pump(const Duration(milliseconds: 100));
+      expect(log, equals(['scrollstart', 'scroll']));
+      tester.pump(const Duration(milliseconds: 1500));
+      expect(log, equals(['scrollstart', 'scroll', 'scroll', 'scrollend']));
+    });
+  });
+
+  test('fling, fling generates one start/end pair', () {
+    testWidgets((WidgetTester tester) {
+      GlobalKey<ScrollableState> scrollKey = new GlobalKey<ScrollableState>();
+      List<String> log = <String>[];
+      tester.pumpWidget(_buildScroller(key: scrollKey, log: log));
+
+      expect(log, equals([]));
+      tester.flingFrom(new Point(100.0, 100.0), new Offset(-50.0, -50.0), 500.0);
+      tester.pump(new Duration(seconds: 1));
+      log.removeWhere((String value) => value == 'scroll');
+      expect(log, equals(['scrollstart']));
+      tester.flingFrom(new Point(100.0, 100.0), new Offset(-50.0, -50.0), 500.0);
+      tester.pump(new Duration(seconds: 1));
+      tester.pump(new Duration(seconds: 1));
+      log.removeWhere((String value) => value == 'scroll');
+      expect(log, equals(['scrollstart', 'scrollend']));
+    });
+  });
 }

packages/flutter_test/lib/src/widget_tester.dart

@@ -78,7 +78,7 @@ class WidgetTester extends Instrumentation {
       super(binding: _SteppedWidgetFlutterBinding.ensureInitialized()) {
     timeDilation = 1.0;
     ui.window.onBeginFrame = null;
-    runApp(new ErrorWidget()); // flush out the last build entirely
+    runApp(new Container(key: new UniqueKey())); // flush out the last build entirely
   }
 
   final FakeAsync async;

packages/flutter_tools/lib/src/commands/create.dart

@@ -128,21 +128,19 @@ All done! In order to run your application, type:
 
   void _renderTemplates(String projectName, String dirPath,
       String flutterPackagesDirectory, { bool renderDriverTest: false }) {
-    String relativePackagesDirectory = path.relative(
-      flutterPackagesDirectory,
-      from: path.join(dirPath, 'pubspec.yaml')
-    );
+    new Directory(dirPath).createSync(recursive: true);
 
-    printStatus('Creating project ${path.basename(projectName)}:');
+    flutterPackagesDirectory = path.normalize(flutterPackagesDirectory);
+    flutterPackagesDirectory = _relativePath(from: dirPath, to: flutterPackagesDirectory);
 
-    new Directory(dirPath).createSync(recursive: true);
+    printStatus('Creating project ${path.basename(projectName)}:');
 
     Map templateContext = <String, dynamic>{
       'projectName': projectName,
       'androidIdentifier': _createAndroidIdentifier(projectName),
       'iosIdentifier': _createUTIIdentifier(projectName),
       'description': description,
-      'flutterPackagesDirectory': relativePackagesDirectory,
+      'flutterPackagesDirectory': flutterPackagesDirectory,
       'androidMinApiLevel': android.minApiLevel
     };
 
@@ -211,3 +209,11 @@ String _validateProjectName(String projectName) {
 
   return null;
 }
+
+String _relativePath({ String from, String to }) {
+  String result = path.relative(to, from: from);
+  // `path.relative()` doesn't always return a correct result: dart-lang/path#12.
+  if (FileSystemEntity.isDirectorySync(path.join(from, result)))
+    return result;
+  return to;
+}

packages/newton/lib/src/simulation.dart

@@ -4,20 +4,18 @@
 
 import 'tolerance.dart';
 
-abstract class Simulatable {
-  /// The current position of the object in the simulation
-  double x(double time);
-
-  /// The current velocity of the object in the simulation
-  double dx(double time);
-}
-
 /// The base class for all simulations. The user is meant to instantiate an
 /// instance of a simulation and query the same for the position and velocity
 /// of the body at a given interval.
-abstract class Simulation implements Simulatable {
+abstract class Simulation {
   Tolerance tolerance = toleranceDefault;
 
+  /// The current position of the object in the simulation
+  double x(double time);
+
+  /// The current velocity of the object in the simulation
+  double dx(double time); // TODO(ianh): remove this; see https://github.com/flutter/flutter/issues/2092
+
   /// Returns if the simulation is done at a given time
   bool isDone(double time);
 }

packages/newton/lib/src/simulation_group.dart

@@ -10,7 +10,8 @@ import 'utils.dart';
 /// must implement the appropriate methods to select the appropriate simulation
 /// at a given time interval. The simulation group takes care to call the `step`
 /// method at appropriate intervals. If more fine grained control over the the
-/// step is necessary, subclasses may override `Simulatable` methods.
+/// step is necessary, subclasses may override the [x], [dx], and [isDone]
+/// methods.
 abstract class SimulationGroup extends Simulation {
 
   /// The currently active simulation

packages/newton/lib/src/spring_simulation.dart

@@ -7,120 +7,154 @@ import 'dart:math' as math;
 import 'simulation.dart';
 import 'utils.dart';
 
-abstract class _SpringSolution implements Simulatable {
-  factory _SpringSolution(
-      SpringDescription desc, double initialPosition, double initialVelocity) {
-    double cmk =
-        desc.damping * desc.damping - 4 * desc.mass * desc.springConstant;
+enum SpringType { unknown, criticallyDamped, underDamped, overDamped }
 
-    if (cmk == 0.0) {
+abstract class _SpringSolution {
+  factory _SpringSolution(
+    SpringDescription desc,
+    double initialPosition,
+    double initialVelocity
+  ) {
+    double cmk = desc.damping * desc.damping - 4 * desc.mass * desc.springConstant;
+    if (cmk == 0.0)
       return new _CriticalSolution(desc, initialPosition, initialVelocity);
-    } else if (cmk > 0.0) {
+    if (cmk > 0.0)
       return new _OverdampedSolution(desc, initialPosition, initialVelocity);
-    } else {
-      return new _UnderdampedSolution(desc, initialPosition, initialVelocity);
-    }
-
-    return null;
+    return new _UnderdampedSolution(desc, initialPosition, initialVelocity);
   }
 
+  double x(double time);
+  double dx(double time);
   SpringType get type;
 }
 
 class _CriticalSolution implements _SpringSolution {
-  final double _r, _c1, _c2;
-
   factory _CriticalSolution(
-      SpringDescription desc, double distance, double velocity) {
+    SpringDescription desc,
+    double distance,
+    double velocity
+  ) {
     final double r = -desc.damping / (2.0 * desc.mass);
     final double c1 = distance;
     final double c2 = velocity / (r * distance);
     return new _CriticalSolution.withArgs(r, c1, c2);
   }
 
-  SpringType get type => SpringType.criticallyDamped;
-
   _CriticalSolution.withArgs(double r, double c1, double c2)
-      : _r = r,
-        _c1 = c1,
-        _c2 = c2;
+    : _r = r,
+      _c1 = c1,
+      _c2 = c2;
 
-  double x(double time) => (_c1 + _c2 * time) * math.pow(math.E, _r * time);
+  final double _r, _c1, _c2;
+
+  double x(double time) {
+    return (_c1 + _c2 * time) * math.pow(math.E, _r * time);
+  }
 
   double dx(double time) {
     final double power = math.pow(math.E, _r * time);
     return _r * (_c1 + _c2 * time) * power + _c2 * power;
   }
+
+  SpringType get type => SpringType.criticallyDamped;
 }
 
 class _OverdampedSolution implements _SpringSolution {
-  final double _r1, _r2, _c1, _c2;
-
   factory _OverdampedSolution(
-      SpringDescription desc, double distance, double velocity) {
-    final double cmk =
-        desc.damping * desc.damping - 4 * desc.mass * desc.springConstant;
-
+    SpringDescription desc,
+    double distance,
+    double velocity
+  ) {
+    final double cmk = desc.damping * desc.damping - 4 * desc.mass * desc.springConstant;
     final double r1 = (-desc.damping - math.sqrt(cmk)) / (2.0 * desc.mass);
     final double r2 = (-desc.damping + math.sqrt(cmk)) / (2.0 * desc.mass);
     final double c2 = (velocity - r1 * distance) / (r2 - r1);
     final double c1 = distance - c2;
-
     return new _OverdampedSolution.withArgs(r1, r2, c1, c2);
   }
 
   _OverdampedSolution.withArgs(double r1, double r2, double c1, double c2)
-      : _r1 = r1,
-        _r2 = r2,
-        _c1 = c1,
-        _c2 = c2;
+    : _r1 = r1,
+      _r2 = r2,
+      _c1 = c1,
+      _c2 = c2;
 
-  SpringType get type => SpringType.overDamped;
+  final double _r1, _r2, _c1, _c2;
 
-  double x(double time) =>
-      (_c1 * math.pow(math.E, _r1 * time) + _c2 * math.pow(math.E, _r2 * time));
+  double x(double time) {
+    return _c1 * math.pow(math.E, _r1 * time) +
+           _c2 * math.pow(math.E, _r2 * time);
+  }
+
+  double dx(double time) {
+    return _c1 * _r1 * math.pow(math.E, _r1 * time) +
+           _c2 * _r2 * math.pow(math.E, _r2 * time);
+  }
 
-  double dx(double time) => (_c1 * _r1 * math.pow(math.E, _r1 * time) +
-      _c2 * _r2 * math.pow(math.E, _r2 * time));
+  SpringType get type => SpringType.overDamped;
 }
 
 class _UnderdampedSolution implements _SpringSolution {
-  final double _w, _r, _c1, _c2;
-
   factory _UnderdampedSolution(
-      SpringDescription desc, double distance, double velocity) {
+    SpringDescription desc,
+    double distance,
+    double velocity
+  ) {
     final double w = math.sqrt(4.0 * desc.mass * desc.springConstant -
-            desc.damping * desc.damping) /
-        (2.0 * desc.mass);
+                     desc.damping * desc.damping) / (2.0 * desc.mass);
     final double r = -(desc.damping / 2.0 * desc.mass);
     final double c1 = distance;
     final double c2 = (velocity - r * distance) / w;
-
     return new _UnderdampedSolution.withArgs(w, r, c1, c2);
   }
 
   _UnderdampedSolution.withArgs(double w, double r, double c1, double c2)
-      : _w = w,
-        _r = r,
-        _c1 = c1,
-        _c2 = c2;
+    : _w = w,
+      _r = r,
+      _c1 = c1,
+      _c2 = c2;
 
-  SpringType get type => SpringType.underDamped;
+  final double _w, _r, _c1, _c2;
 
-  double x(double time) => math.pow(math.E, _r * time) *
-      (_c1 * math.cos(_w * time) + _c2 * math.sin(_w * time));
+  double x(double time) {
+    return math.pow(math.E, _r * time) *
+           (_c1 * math.cos(_w * time) + _c2 * math.sin(_w * time));
+  }
 
   double dx(double time) {
     final double power = math.pow(math.E, _r * time);
     final double cosine = math.cos(_w * time);
     final double sine = math.sin(_w * time);
-
-    return power * (_c2 * _w * cosine - _c1 * _w * sine) +
-        _r * power * (_c2 * sine + _c1 * cosine);
+    return      power * (_c2 * _w * cosine - _c1 * _w * sine) +
+           _r * power * (_c2 *      sine   + _c1 *      cosine);
   }
+
+  SpringType get type => SpringType.underDamped;
 }
 
 class SpringDescription {
+  SpringDescription({
+    this.mass,
+    this.springConstant,
+    this.damping
+  }) {
+    assert(mass != null);
+    assert(springConstant != null);
+    assert(damping != null);
+  }
+
+  /// Create a spring given the mass, spring constant and the damping ratio. The
+  /// damping ratio is especially useful trying to determing the type of spring
+  /// to create. A ratio of 1.0 creates a critically damped spring, > 1.0
+  /// creates an overdamped spring and < 1.0 an underdamped one.
+  SpringDescription.withDampingRatio({
+    double mass,
+    double springConstant,
+    double ratio: 1.0
+  }) : mass = mass,
+       springConstant = springConstant,
+       damping = ratio * 2.0 * math.sqrt(mass * springConstant);
+
   /// The mass of the spring (m)
   final double mass;
 
@@ -131,41 +165,23 @@ class SpringDescription {
   /// Not to be confused with the damping ratio. Use the separate
   /// constructor provided for this purpose
   final double damping;
-
-  SpringDescription(
-      { this.mass, this.springConstant, this.damping }
-  ) {
-    assert(mass != null);
-    assert(springConstant != null);
-    assert(damping != null);
-  }
-
-  /// Create a spring given the mass, spring constant and the damping ratio. The
-  /// damping ratio is especially useful trying to determing the type of spring
-  /// to create. A ratio of 1.0 creates a critically damped spring, > 1.0
-  /// creates an overdamped spring and < 1.0 an underdamped one.
-  SpringDescription.withDampingRatio(
-      {double mass, double springConstant, double ratio: 1.0})
-      : mass = mass,
-        springConstant = springConstant,
-        damping = ratio * 2.0 * math.sqrt(mass * springConstant);
 }
 
-enum SpringType { unknown, criticallyDamped, underDamped, overDamped, }
-
 /// Creates a spring simulation. Depending on the spring description, a
 /// critically, under or overdamped spring will be created.
 class SpringSimulation extends Simulation {
-  final double _endPosition;
-
-  final _SpringSolution _solution;
-
   /// A spring description with the provided spring description, start distance,
   /// end distance and velocity.
   SpringSimulation(
-      SpringDescription desc, double start, double end, double velocity)
-      : this._endPosition = end,
-        _solution = new _SpringSolution(desc, start - end, velocity);
+    SpringDescription desc,
+    double start,
+    double end,
+    double velocity
+  ) : _endPosition = end,
+      _solution = new _SpringSolution(desc, start - end, velocity);
+
+  final double _endPosition;
+  final _SpringSolution _solution;
 
   SpringType get type => _solution.type;
 
@@ -182,8 +198,12 @@ class SpringSimulation extends Simulation {
 /// A SpringSimulation where the value of x() is guaranteed to have exactly the
 /// end value when the simulation isDone().
 class ScrollSpringSimulation extends SpringSimulation {
-  ScrollSpringSimulation(SpringDescription desc, double start, double end, double velocity)
-    : super(desc, start, end, velocity);
+  ScrollSpringSimulation(
+    SpringDescription desc,
+    double start,
+    double end,
+    double velocity
+  ) : super(desc, start, end, velocity);
 
   double x(double time) => isDone(time) ? _endPosition : super.x(time);
 }