Make the Icon component use Image

Even though Image doesn't yet know how to handle
relative urls.

TBR=jackson@google.com

Review URL: https://codereview.chromium.org/1157163009

sdk/lib/framework/components2/icon.dart

@@ -10,7 +10,7 @@ import '../rendering/box.dart';
 const String kAssetBase = '/packages/sky/assets/material-design-icons';
 
 class Icon extends Component {
-  int size;
+  double size;
   String type;
 
   Icon({
@@ -27,12 +27,9 @@ class Icon extends Component {
       category = parts[0];
       subtype = parts[1];
     }
-    // TODO(jackson): Use a real image.
-    return new Container(
-      desiredSize: new sky.Size(size.toDouble(), size.toDouble()),
-      decoration: new BoxDecoration(backgroundColor: const sky.Color(0xFFFF0000))
+    return new Image(
+      size: new sky.Size(size, size),
+      src: '${kAssetBase}/${category}/2x_web/ic_${subtype}_${size}dp.png'
     );
-    // return new Image(width: size, height: size,
-    //   src: '${kAssetBase}/${category}/2x_web/ic_${subtype}_${size}dp.png');
   }
 }

sdk/lib/framework/components2/icon_button.dart

@@ -16,7 +16,7 @@ class IconButton extends Component {
   UINode build() {
     return new EventListenerNode(
       new Padding(
-        child: new Icon(type: icon, size: 24),
+        child: new Icon(type: icon, size: 24.0),
         padding: const EdgeDims.all(8.0)),
       onGestureTap: onGestureTap);
   }

sdk/lib/framework/components2/menu_item.dart

@@ -44,7 +44,7 @@ class MenuItem extends ButtonBase {
           children: [
             new StyleNode(
               new Icon(
-                size: 24,
+                size: 24.0,
                 type: "${icon}_grey600"
               ),
               _iconStyle