Refactor fn2.dart, since it breached our 1000-line threshold.

sky/sdk/lib/framework/components2/button_base.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 '../fn2.dart';
-
-abstract class ButtonBase extends Component {
-
-  ButtonBase({ Object key, this.highlight: false }) : super(key: key);
-
-  bool highlight;
-
-  void syncFields(ButtonBase source) {
-    highlight = source.highlight;
-  }
-
-  void _handlePointerDown(_) {
-    setState(() {
-      highlight = true;
-    });
-  }
-  void _handlePointerUp(_) {
-    setState(() {
-      highlight = false;
-    });
-  }
-  void _handlePointerCancel(_) {
-    setState(() {
-      highlight = false;
-    });
-  }
-
-  UINode build() {
-    return new EventListenerNode(
-      buildContent(),
-      onPointerDown: _handlePointerDown,
-      onPointerUp: _handlePointerUp,
-      onPointerCancel: _handlePointerCancel
-    );
-  }
-
-  UINode buildContent();
-
-}