Add a basic popup menu widget

sky/framework/components/popup_menu.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 '../fn.dart';
+import '../theme/colors.dart';
+import 'material.dart';
+
+class PopupMenu extends Component {
+  static final Style _style = new Style('''
+    border-radius: 2px;
+    background-color: ${Grey[50]};'''
+  );
+
+  List<Node> children;
+  int level;
+
+  PopupMenu({ Object key, this.children, this.level }) : super(key: key);
+
+  Node build() {
+    return new Material(
+      style: _style,
+      children: children,
+      level: level
+    );
+  }
+}