Automatically fill in the menu button when there's a drawer

Also, fill in a back arrow when we can go back.

Fixes #699

examples/address_book/lib/main.dart

@@ -36,9 +36,8 @@ class Field extends StatelessComponent {
 class AddressBookHome extends StatelessComponent {
   Widget buildToolBar(BuildContext context) {
     return new ToolBar(
-        left: new IconButton(icon: "navigation/arrow_back"),
-        right: <Widget>[new IconButton(icon: "navigation/check")]
-      );
+      right: <Widget>[new IconButton(icon: "navigation/check")]
+    );
   }
 
   Widget buildFloatingActionButton(BuildContext context) {

examples/fitness/lib/feed.dart

@@ -55,7 +55,6 @@ class FeedFragment extends StatefulComponent {
 
 class FeedFragmentState extends State<FeedFragment> {
   FitnessMode _fitnessMode = FitnessMode.feed;
-  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
 
   void _handleFitnessModeChange(FitnessMode value) {
     setState(() {
@@ -104,9 +103,6 @@ class FeedFragmentState extends State<FeedFragment> {
 
   Widget buildToolBar() {
     return new ToolBar(
-      left: new IconButton(
-        icon: "navigation/menu",
-        onPressed: () => _scaffoldKey.currentState?.openDrawer()),
       center: new Text(fitnessModeTitle)
     );
   }
@@ -207,7 +203,6 @@ class FeedFragmentState extends State<FeedFragment> {
 
   Widget build(BuildContext context) {
     return new Scaffold(
-      key: _scaffoldKey,
       toolBar: buildToolBar(),
       body: buildBody(),
       floatingActionButton: buildFloatingActionButton(),

examples/fitness/lib/settings.dart

@@ -26,10 +26,6 @@ class SettingsFragmentState extends State<SettingsFragment> {
 
   Widget buildToolBar() {
     return new ToolBar(
-      left: new IconButton(
-        icon: "navigation/arrow_back",
-        onPressed: () => Navigator.pop(context)
-      ),
       center: new Text('Settings')
     );
   }

examples/material_gallery/lib/gallery_page.dart

@@ -17,8 +17,6 @@ class GalleryPage extends StatefulComponent {
 }
 
 class _GalleryPageState extends State<GalleryPage> {
-  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
-
   Widget _buildDrawer() {
     List<Widget> items = <Widget>[
       new DrawerHeader(child: new Text('Material demos')),
@@ -53,12 +51,7 @@ class _GalleryPageState extends State<GalleryPage> {
 
   Widget build(BuildContext context) {
     return new Scaffold(
-      key: _scaffoldKey,
       toolBar: new ToolBar(
-        left: new IconButton(
-          icon: 'navigation/menu',
-          onPressed: () { _scaffoldKey.currentState?.openDrawer(); }
-        ),
         center: new Text(config.active?.title ?? 'Material gallery'),
         tabBar: _buildTabBar()
       ),

examples/stocks/lib/stock_home.dart

@@ -147,10 +147,6 @@ class StockHomeState extends State<StockHome> {
   Widget buildToolBar() {
     return new ToolBar(
       elevation: 0,
-      left: new IconButton(
-        icon: "navigation/menu",
-        onPressed: () => _scaffoldKey.currentState?.openDrawer()
-      ),
       center: new Text('Stocks'),
       right: <Widget>[
         new IconButton(
@@ -230,7 +226,7 @@ class StockHomeState extends State<StockHome> {
   Widget buildSearchBar() {
     return new ToolBar(
       left: new IconButton(
-        icon: "navigation/arrow_back",
+        icon: 'navigation/arrow_back',
         colorFilter: new ColorFilter.mode(Theme.of(context).accentColor, ui.TransferMode.srcATop),
         onPressed: _handleSearchEnd
       ),

examples/stocks/lib/stock_settings.dart

@@ -70,10 +70,6 @@ class StockSettingsState extends State<StockSettings> {
 
   Widget buildToolBar(BuildContext context) {
     return new ToolBar(
-      left: new IconButton(
-        icon: 'navigation/arrow_back',
-        onPressed: () => Navigator.pop(context)
-      ),
       center: new Text('Settings')
     );
   }

examples/stocks/lib/stock_symbol_viewer.dart

@@ -55,12 +55,6 @@ class StockSymbolPage extends StatelessComponent {
   Widget build(BuildContext context) {
     return new Scaffold(
       toolBar: new ToolBar(
-        left: new IconButton(
-          icon: 'navigation/arrow_back',
-          onPressed: () {
-            Navigator.pop(context);
-          }
-        ),
         center: new Text(stock.name)
       ),
       body: new Block(<Widget>[

examples/widgets/card_collection.dart

@@ -46,8 +46,6 @@ class CardCollectionState extends State<CardCollection> {
   InvalidatorCallback _invalidator;
   Size _cardCollectionSize = new Size(200.0, 200.0);
 
-  GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
-
   void _initVariableSizedCardModels() {
     List<double> cardHeights = <double>[
       48.0, 63.0, 82.0, 146.0, 60.0, 55.0, 84.0, 96.0, 50.0,
@@ -268,7 +266,6 @@ class CardCollectionState extends State<CardCollection> {
 
   Widget _buildToolBar() {
     return new ToolBar(
-      left: new IconButton(icon: "navigation/menu", onPressed: () => _scaffoldKey.currentState?.openDrawer()),
       right: <Widget>[
         new Text(_dismissDirectionText(_dismissDirection))
       ],

examples/widgets/pageable_list.dart

@@ -41,8 +41,6 @@ class PageableListAppState extends State<PageableListApp> {
   ScrollDirection scrollDirection = ScrollDirection.horizontal;
   bool itemsWrap = false;
 
-  GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
-
   void updatePageSize(Size newSize) {
     setState(() {
       pageSize = newSize;
@@ -114,7 +112,6 @@ class PageableListAppState extends State<PageableListApp> {
 
   Widget _buildToolBar() {
     return new ToolBar(
-      left: new IconButton(icon: "navigation/menu", onPressed: () => _scaffoldKey.currentState?.openDrawer()),
       center: new Text('PageableList'),
       right: <Widget>[
         new Text(scrollDirection == ScrollDirection.horizontal ? "horizontal" : "vertical")

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

@@ -16,6 +16,7 @@ import 'material.dart';
 import 'snack_bar.dart';
 import 'tool_bar.dart';
 import 'drawer.dart';
+import 'icon_button.dart';
 
 const double _kFloatingActionButtonMargin = 16.0; // TODO(hmuller): should be device dependent
 
@@ -257,8 +258,37 @@ class ScaffoldState extends State<Scaffold> {
       children.add(new LayoutId(child: child, id: childId));
   }
 
+  bool _shouldShowBackArrow;
+
+  Widget get _modifiedToolBar {
+    ToolBar toolBar = config.toolBar;
+    if (toolBar == null)
+      return null;
+    EdgeDims padding = new EdgeDims.only(top: ui.window.padding.top);
+    Widget left = toolBar.left;
+    if (left == null) {
+      if (config.drawer != null) {
+        left = new IconButton(
+          icon: 'navigation/menu',
+          onPressed: openDrawer
+        );
+      } else {
+        _shouldShowBackArrow ??= Navigator.canPop(context);
+        if (_shouldShowBackArrow) {
+          left = new IconButton(
+            icon: 'navigation/arrow_back',
+            onPressed: () => Navigator.pop(context)
+          );
+        }
+      }
+    }
+    return toolBar.copyWith(
+      padding: padding,
+      left: left
+    );
+  }
+
   Widget build(BuildContext context) {
-    final Widget paddedToolBar = config.toolBar?.withPadding(new EdgeDims.only(top: ui.window.padding.top));
     final Widget materialBody = config.body != null ? new Material(child: config.body) : null;
 
     if (_snackBars.length > 0) {
@@ -274,7 +304,7 @@ class ScaffoldState extends State<Scaffold> {
 
     final List<LayoutId>children = new List<LayoutId>();
     _addIfNonNull(children, materialBody, _Child.body);
-    _addIfNonNull(children, paddedToolBar, _Child.toolBar);
+    _addIfNonNull(children, _modifiedToolBar, _Child.toolBar);
 
     if (_currentBottomSheet != null ||
         (_dismissedBottomSheets != null && _dismissedBottomSheets.isNotEmpty)) {

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

@@ -36,18 +36,28 @@ class ToolBar extends StatelessComponent {
   final TextTheme textTheme;
   final EdgeDims padding;
 
-  ToolBar withPadding(EdgeDims newPadding, { Key fallbackKey }) {
+  ToolBar copyWith({
+    Key key,
+    Widget left,
+    Widget center,
+    List<Widget> right,
+    Widget bottom,
+    int elevation,
+    Color backgroundColor,
+    TextTheme textTheme,
+    EdgeDims padding
+  }) {
     return new ToolBar(
-      key: key ?? fallbackKey,
-      left: left,
-      center: center,
-      right: right,
-      bottom: bottom,
-      tabBar: tabBar,
-      elevation: elevation,
-      backgroundColor: backgroundColor,
-      textTheme: textTheme,
-      padding: newPadding
+      key: key ?? this.key,
+      left: left ?? this.left,
+      center: center ?? this.center,
+      right: right ?? this.right,
+      bottom: bottom ?? this.bottom,
+      tabBar: tabBar ?? this.tabBar,
+      elevation: elevation ?? this.elevation,
+      backgroundColor: backgroundColor ?? this.backgroundColor,
+      textTheme: textTheme ?? this.textTheme,
+      padding: padding ?? this.padding
     );
   }