Make RaisedButton support being disabled.

R=jackson@google.com

Review URL: https://codereview.chromium.org/1179943005.

examples/widgets/container.dart

@@ -24,10 +24,19 @@ class ContainerApp extends App {
           key: 'b',
           decoration: new BoxDecoration(backgroundColor: const Color(0xFFFFFF00)),
           padding: new EdgeDims.symmetric(horizontal: 50.0, vertical: 75.0),
-          child: new RaisedButton(
-            child: new Text('PRESS ME'),
-            onPressed: () => print("Hello World")
-          )
+          child: new Flex([
+            new RaisedButton(
+              key: 'b1',
+              child: new Text('PRESS ME'),
+              onPressed: () => print("Hello World")
+            ),
+            new RaisedButton(
+              key: 'b2',
+              child: new Text('DISABLED'),
+              onPressed: () => print("Hello World"),
+              enabled: false
+            )
+          ])
         ),
         new FlexExpandingChild(
           new Container(