Add DefaultAssetBundle

Now AssetImage will look in the default bundle if you don't supply an explicit
bundle.

Fixes #680

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

@@ -2,22 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
 import 'theme.dart';
 import 'icon_theme.dart';
 import 'icon_theme_data.dart';
 
-AssetBundle _initIconBundle() {
-  if (rootBundle != null)
-    return rootBundle;
-  const String _kAssetBase = '/packages/material_design_icons/icons/';
-  return new NetworkAssetBundle(Uri.base.resolve(_kAssetBase));
-}
-
-final AssetBundle _iconBundle = _initIconBundle();
-
 class Icon extends StatelessComponent {
   Icon({
     Key key,
@@ -66,7 +56,6 @@ class Icon extends StatelessComponent {
     String density = 'drawable-xxhdpi';
     String colorSuffix = _getColorSuffix(context);
     return new AssetImage(
-      bundle: _iconBundle,
       name: '$category/$density/ic_${subtype}_${colorSuffix}_${size}dp.png',
       width: size.toDouble(),
       height: size.toDouble(),

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

@@ -19,6 +19,15 @@ const TextStyle _errorTextStyle = const TextStyle(
   decorationStyle: TextDecorationStyle.double
 );
 
+AssetBundle _initDefaultBundle() {
+  if (rootBundle != null)
+    return rootBundle;
+  const String _kAssetBase = '/packages/material_design_icons/icons/';
+  return new NetworkAssetBundle(Uri.base.resolve(_kAssetBase));
+}
+
+final AssetBundle _defaultBundle = _initDefaultBundle();
+
 class MaterialApp extends StatefulComponent {
   MaterialApp({
     Key key,
@@ -68,12 +77,15 @@ class _MaterialAppState extends State<MaterialApp> {
       data: config.theme ?? new ThemeData.fallback(),
       child: new DefaultTextStyle(
         style: _errorTextStyle,
-        child: new Title(
-          title: config.title,
-          child: new Navigator(
-            key: _navigator,
-            routes: config.routes,
-            onGenerateRoute: config.onGenerateRoute
+        child: new DefaultAssetBundle(
+          bundle: _defaultBundle,
+          child: new Title(
+            title: config.title,
+            child: new Navigator(
+              key: _navigator,
+              routes: config.routes,
+              onGenerateRoute: config.onGenerateRoute
+            )
           )
         )
       )

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

@@ -1017,6 +1017,26 @@ class NetworkImage extends StatelessComponent {
   }
 }
 
+class DefaultAssetBundle extends InheritedWidget {
+  DefaultAssetBundle({
+    Key key,
+    this.bundle,
+    Widget child
+  }) : super(key: key, child: child) {
+    assert(bundle != null);
+    assert(child != null);
+  }
+
+  final AssetBundle bundle;
+
+  static AssetBundle of(BuildContext context) {
+    DefaultAssetBundle result = context.inheritedWidgetOfType(DefaultAssetBundle);
+    return result?.bundle;
+  }
+
+  bool updateShouldNotify(DefaultAssetBundle old) => bundle != old.bundle;
+}
+
 class AssetImage extends StatelessComponent {
   AssetImage({
     Key key,
@@ -1041,7 +1061,7 @@ class AssetImage extends StatelessComponent {
 
   Widget build(BuildContext context) {
     return new ImageListener(
-      image: bundle.loadImage(name),
+      image: (bundle ?? DefaultAssetBundle.of(context)).loadImage(name),
       width: width,
       height: height,
       colorFilter: colorFilter,