Add FlatButton

sky/sdk/lib/widgets/flat_button.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';
+import 'material_button.dart';
+
+class FlatButton extends MaterialButton {
+  FlatButton({
+    String key,
+    Widget child,
+    bool enabled: true,
+    Function onPressed
+  }) : super(key: key, child: child, enabled: enabled, onPressed: onPressed);
+
+  Color get color => null;
+  int get level => null;
+}