Hoist knowledge of image_cache into widgets/basic.dart

sky/sdk/lib/widgets/icon.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'basic.dart';
 
 // TODO(eseidel): This should use package:.
 const String kAssetBase = '/packages/sky/assets/material-design-icons';
 
 class Icon extends Component {
@@ skipping 11 matching lines @@
     String category = '';
     String subtype = '';
     List<String> parts = type.split('/');
     if (parts.length == 2) {
       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(
+    return new NetworkImage(
       size: new Size(size.toDouble(), size.toDouble()),
       src: '${kAssetBase}/${category}/${density}/ic_${subtype}_${size}dp.png'
     );
   }
 
 }