Hoist knowledge of image_cache into widgets/basic.dart

sky/sdk/lib/widgets/icon.dart

Index: sky/sdk/lib/widgets/icon.dart
diff --git a/sky/sdk/lib/widgets/icon.dart b/sky/sdk/lib/widgets/icon.dart
index 1d1e7a08c257067683e8e005b58304ad0d699c27..5bb3fac8032b8fd7f47fe15b873d8370f531e76c 100644
--- a/sky/sdk/lib/widgets/icon.dart
+++ b/sky/sdk/lib/widgets/icon.dart
@@ -29,7 +29,7 @@ class Icon extends Component {
     // TODO(eseidel): This clearly isn't correct.  Not sure what would be.
     // Should we use the ios images on ios?
     String density = 'drawable-xxhdpi';
-    return new Image(
+    return new NetworkImage(
       size: new Size(size.toDouble(), size.toDouble()),
       src: '${kAssetBase}/${category}/${density}/ic_${subtype}_${size}dp.png'
     );