[Effen] Make the drawer not be included in the build output when the drawer is not shown.

sky/framework/animation/animated_value.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 'curves.dart';
 import 'dart:async';
 import 'generators.dart';
 
+typedef void Callback ();
+
 class AnimatedValue {
-  StreamController _controller = new StreamController(sync: true);
+  StreamController _controller = new StreamController.broadcast(sync: true);
   AnimationGenerator _animation;
   Completer _completer;
   double _value;
 
-  AnimatedValue(double initial) {
+  AnimatedValue(double initial, { Callback onChange }) {
     _value = initial;
+    _onChange = onChange;
   }
+  Callback _onChange;
 
   // A stream of change in value from |initial|. The stream does not
   // contain the initial value. Consumers should check the initial value via
   // the |value| accessor.
   Stream<double> get onValueChanged => _controller.stream;
 
   double get value => _value;
 
   void set value(double value) {
     stop();
    _setValue(value);
   }
 
   bool get isAnimating => _animation != null;
 
   void _setValue(double value) {
     _value = value;
     _controller.add(_value);
+    if (_onChange != null)
+      _onChange();
   }
 
   void _done() {
     _animation = null;
     if (_completer == null)
       return;
     Completer completer = _completer;
     _completer = null;
     completer.complete(_value);
   }
@@ skipping 21 matching lines @@
     return _completer.future;
   }
 
   double get remainingTime {
     if (_animation == null)
       return 0.0;
     return _animation.remainingTime;
   }
 
 }