Use drawable_xxhdpi instead of 2x_web

sky/sdk/lib/framework/widgets/icon.dart

Index: sky/sdk/lib/framework/widgets/icon.dart
diff --git a/sky/sdk/lib/framework/widgets/icon.dart b/sky/sdk/lib/framework/widgets/icon.dart
index abd7b6cefd7a4ae5b51864781b397d8006c6403c..35f01938b4f5ade86b6d2bc37776ea03c49901ed 100644
--- a/sky/sdk/lib/framework/widgets/icon.dart
+++ b/sky/sdk/lib/framework/widgets/icon.dart
@@ -26,9 +26,12 @@ class Icon extends Component {
       category = parts[0];
       subtype = parts[1];
     }
+    // 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(
       size: new Size(size.toDouble(), size.toDouble()),
-      src: '${kAssetBase}/${category}/2x_web/ic_${subtype}_${size}dp.png'
+      src: '${kAssetBase}/${category}/${density}/ic_${subtype}_${size}dp.png'
     );
   }