Reimplement the theme transition animation by actually animating the Theme.

As part of this:
 - A lot of classes got new lerp functions, including e.g. TextStyle.
 - Theme's constructor story got overhauled. You can now configure
   everything if you really want to, and we're better about defaults.
 - Material no longer automatically animates its background color.
   (It still does for its shadow.)
 - Tabs try to get the indicator color from the theme.
 - The fields in ThemeData got reordered for sanity.
 - Theme.== and Theme.hashCode got fixed.
 - Typography got a bit of a spring cleaning.

Fixes #613.

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

@@ -8,6 +8,12 @@ class IconThemeData {
   const IconThemeData({ this.color });
   final IconThemeColor color;
 
+  static IconThemeData lerp(IconThemeData begin, IconThemeData end, double t) {
+    return new IconThemeData(
+      color: t < 0.5 ? begin.color : end.color
+    );
+  }
+
   bool operator ==(dynamic other) {
     if (other is! IconThemeData)
       return false;

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

@@ -160,12 +160,18 @@ class _MaterialState extends State<Material> {
         curve: Curves.ease,
         duration: kThemeChangeDuration,
         decoration: new BoxDecoration(
-          backgroundColor: backgroundColor,
           borderRadius: kMaterialEdges[config.type],
           boxShadow: config.elevation == 0 ? null : elevationToShadow[config.elevation],
           shape: config.type == MaterialType.circle ? BoxShape.circle : BoxShape.rectangle
         ),
-        child: contents
+        child: new Container(
+          decoration: new BoxDecoration(
+            borderRadius: kMaterialEdges[config.type],
+            backgroundColor: backgroundColor,
+            shape: config.type == MaterialType.circle ? BoxShape.circle : BoxShape.rectangle
+          ),
+          child: contents
+        )
       );
     }
     return contents;

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

@@ -141,8 +141,9 @@ class _MaterialAppState extends State<MaterialApp> implements BindingObserver {
       data: new MediaQueryData(size: _size),
       child: new LocaleQuery(
         data: _localeData,
-        child: new Theme(
+        child: new AnimatedTheme(
           data: theme,
+          duration: kThemeAnimationDuration,
           child: new DefaultTextStyle(
             style: _errorTextStyle,
             child: new DefaultAssetBundle(

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

@@ -717,9 +717,17 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
 
     ThemeData themeData = Theme.of(context);
     Color backgroundColor = Material.of(context).color;
-    Color indicatorColor = themeData.accentColor;
-    if (indicatorColor == backgroundColor)
+    Color indicatorColor = themeData.indicatorColor;
+    if (indicatorColor == backgroundColor) {
+      // ThemeData tries to avoid this by having indicatorColor avoid being the
+      // primaryColor. However, it's possible that the tab strip is on a
+      // Material that isn't the primaryColor. In that case, if the indicator
+      // color ends up clashing, then this overrides it. When that happens,
+      // automatic transitions of the theme will likely look ugly as the
+      // indicator color suddenly snaps to white at one end, but it's not clear
+      // how to avoid that any further.
       indicatorColor = Colors.white;
+    }
 
     TextStyle textStyle = themeData.primaryTextTheme.body1;
     IconThemeData iconTheme = themeData.primaryIconTheme;

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

@@ -2,12 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/animation.dart';
 import 'package:flutter/widgets.dart';
 
 import 'theme_data.dart';
 
 export 'theme_data.dart' show ThemeData, ThemeBrightness;
 
+const kThemeAnimationDuration = const Duration(milliseconds: 200);
+
 class Theme extends InheritedWidget {
   Theme({
     Key key,
@@ -37,3 +40,55 @@ class Theme extends InheritedWidget {
     description.add('$data');
   }
 }
+
+/// An animated value that interpolates [BoxConstraint]s.
+class AnimatedThemeDataValue extends AnimatedValue<ThemeData> {
+  AnimatedThemeDataValue(ThemeData begin, { ThemeData end, Curve curve, Curve reverseCurve })
+    : super(begin, end: end, curve: curve, reverseCurve: reverseCurve);
+
+  ThemeData lerp(double t) => ThemeData.lerp(begin, end, t);
+}
+
+/// Animated version of [Theme] which automatically transitions the colours,
+/// etc, over a given duration whenever the given theme changes.
+class AnimatedTheme extends AnimatedWidgetBase {
+  AnimatedTheme({
+    Key key,
+    this.data,
+    Curve curve: Curves.linear,
+    Duration duration,
+    this.child
+  }) : super(key: key, curve: curve, duration: duration) {
+    assert(child != null);
+    assert(data != null);
+  }
+
+  final ThemeData data;
+
+  final Widget child;
+
+  _AnimatedThemeState createState() => new _AnimatedThemeState();
+}
+
+class _AnimatedThemeState extends AnimatedWidgetBaseState<AnimatedTheme> {
+  AnimatedThemeDataValue _data;
+
+  void forEachVariable(VariableVisitor visitor) {
+    // TODO(ianh): Use constructor tear-offs when it becomes possible
+    _data = visitor(_data, config.data, (dynamic value) => new AnimatedThemeDataValue(value));
+    assert(_data != null);
+  }
+
+  Widget build(BuildContext context) {
+    return new Theme(
+      child: config.child,
+      data: _data.value
+    );
+  }
+
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    if (_data != null)
+      description.add('$_data');
+  }
+}

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

@@ -4,8 +4,6 @@
 
 // See http://www.google.com/design/spec/style/typography.html
 
-import 'dart:ui' show Color;
-
 import 'package:flutter/painting.dart';
 
 import 'colors.dart';
@@ -13,8 +11,11 @@ import 'colors.dart';
 // TODO(eseidel): Font weights are supposed to be language relative!
 // TODO(jackson): Baseline should be language relative!
 // These values are for English-like text.
+// TODO(ianh): There's no font-family specified here.
 class TextTheme {
 
+  const TextTheme._(this.display4, this.display3, this.display2, this.display1, this.headline, this.title, this.subhead, this.body2, this.body1, this.caption, this.button);
+
   const TextTheme._black()
     : display4 = const TextStyle(inherit: false, fontSize: 112.0, fontWeight: FontWeight.w100, color: Colors.black54, textBaseline: TextBaseline.alphabetic),
       display3 = const TextStyle(inherit: false, fontSize:  56.0, fontWeight: FontWeight.w400, color: Colors.black54, textBaseline: TextBaseline.alphabetic),
@@ -53,26 +54,26 @@ class TextTheme {
   final TextStyle caption;
   final TextStyle button;
 
+  static TextTheme lerp(TextTheme begin, TextTheme end, double t) {
+    return new TextTheme._(
+      TextStyle.lerp(begin.display4, end.display4, t),
+      TextStyle.lerp(begin.display3, end.display3, t),
+      TextStyle.lerp(begin.display2, end.display2, t),
+      TextStyle.lerp(begin.display1, end.display1, t),
+      TextStyle.lerp(begin.headline, end.headline, t),
+      TextStyle.lerp(begin.title, end.title, t),
+      TextStyle.lerp(begin.subhead, end.subhead, t),
+      TextStyle.lerp(begin.body2, end.body2, t),
+      TextStyle.lerp(begin.body1, end.body1, t),
+      TextStyle.lerp(begin.caption, end.caption, t),
+      TextStyle.lerp(begin.button, end.button, t)
+    );
+  }
+
 }
 
 class Typography {
   Typography._();
-
   static const TextTheme black = const TextTheme._black();
   static const TextTheme white = const TextTheme._white();
-
-  // TODO(abarth): Maybe this should be hard-coded in Scaffold?
-  static const String typeface = 'font-family: sans-serif';
-
-  // TODO(ianh): Remove this when we remove fn2, now that it's hard-coded in App.
-  static const TextStyle error = const TextStyle(
-    color: const Color(0xD0FF0000),
-    fontFamily: 'monospace',
-    fontSize: 48.0,
-    fontWeight: FontWeight.w900,
-    textAlign: TextAlign.right,
-    decoration: TextDecoration.underline,
-    decorationColor: const Color(0xFFFF00),
-    decorationStyle: TextDecorationStyle.double
-  );
 }

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

@@ -127,6 +127,31 @@ class TextStyle {
     );
   }
 
+  /// Interpolate between two text styles.
+  ///
+  /// This will not work well if the styles don't set the same fields.
+  static TextStyle lerp(TextStyle begin, TextStyle end, double t) {
+    assert(begin.inherit == end.inherit);
+    return new TextStyle(
+      inherit: end.inherit,
+      color: Color.lerp(begin.color, end.color, t),
+      fontFamily: t < 0.5 ? begin.fontFamily : end.fontFamily,
+      fontSize: ui.lerpDouble(begin.fontSize ?? end.fontSize, end.fontSize ?? begin.fontSize, t),
+      // TODO(ianh): Replace next line with "fontWeight: FontWeight.lerp(begin.fontWeight, end.fontWeight, t)," once engine is revved
+      fontWeight: FontWeight.values[ui.lerpDouble(begin?.fontWeight.index ?? FontWeight.normal.index, end?.fontWeight.index ?? FontWeight.normal.index, t.clamp(0.0, 1.0)).round()],
+      fontStyle: t < 0.5 ? begin.fontStyle : end.fontStyle,
+      letterSpacing: ui.lerpDouble(begin.letterSpacing ?? end.letterSpacing, end.letterSpacing ?? begin.letterSpacing, t),
+      wordSpacing: ui.lerpDouble(begin.wordSpacing ?? end.wordSpacing, end.wordSpacing ?? begin.wordSpacing, t),
+      textAlign: t < 0.5 ? begin.textAlign : end.textAlign,
+      textBaseline: t < 0.5 ? begin.textBaseline : end.textBaseline,
+      height: ui.lerpDouble(begin.height ?? end.height, end.height ?? begin.height, t),
+      decoration: t < 0.5 ? begin.decoration : end.decoration,
+      decorationColor: Color.lerp(begin.decorationColor, end.decorationColor, t),
+      decorationStyle: t < 0.5 ? begin.decorationStyle : end.decorationStyle
+    );
+  }
+  
+
   ui.TextStyle get textStyle {
     return new ui.TextStyle(
       color: color,

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

@@ -317,7 +317,7 @@ class _AnimatedContainerState extends AnimatedWidgetBaseState<AnimatedContainer>
 }
 
 /// Animated version of [Positioned] which automatically transitions the child's
-/// position over a given duration whenever the given positon changes.
+/// position over a given duration whenever the given position changes.
 ///
 /// Only works if it's the child of a [Stack].
 class AnimatedPositioned extends AnimatedWidgetBase {