[Effen] Add AnimatedComponent base class

sky/framework/animation/animated_value.dart

Index: sky/framework/animation/animated_value.dart
diff --git a/sky/framework/animation/animated_value.dart b/sky/framework/animation/animated_value.dart
index faf17b6a82b901a7bc9e3d45253a3cfa0cf36cf6..0db6f90da0302976452fb51998199bffef52f332 100644
--- a/sky/framework/animation/animated_value.dart
+++ b/sky/framework/animation/animated_value.dart
@@ -13,9 +13,12 @@ class AnimatedValue {
   double _value;
 
   AnimatedValue(double initial) {
-    value = initial;
+    _value = initial;
   }
 
+  // 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;
@@ -55,28 +58,3 @@ class AnimatedValue {
     });
   }
 }
-
-class AnimatedValueListener {
-  final Component _component;
-  final AnimatedValue _value;
-  StreamSubscription<double> _subscription;
-
-  AnimatedValueListener(this._component, this._value);
-
-  double get value => _value == null ? null : _value.value;
-
-  void ensureListening() {
-    if (_subscription != null || _value == null)
-      return;
-    _subscription = _value.onValueChanged.listen((_) {
-      _component.scheduleBuild();
-    });
-  }
-
-  void stopListening() {
-    if (_subscription == null)
-      return;
-    _subscription.cancel();
-    _subscription = null;
-  }
-}