Kill onDidMount()/onDidUnmount() in favour of just overriding the relevant methods.

sky/sdk/lib/framework/components2/animated_component.dart

Index: sky/sdk/lib/framework/components2/animated_component.dart
diff --git a/sky/sdk/lib/framework/components2/animated_component.dart b/sky/sdk/lib/framework/components2/animated_component.dart
index 4cc91e01d0c981dd4c8d4b6dfb36aaa922b6e995..9a02fd531dddd9e5b4ea10c6fea0e09382df625e 100644
--- a/sky/sdk/lib/framework/components2/animated_component.dart
+++ b/sky/sdk/lib/framework/components2/animated_component.dart
@@ -8,27 +8,44 @@ import 'dart:async';
 
 typedef void SetterFunction(double value);
 
+class _AnimationEntry {
+  _AnimationEntry(this.value, this.setter);
+  final AnimatedValue value;
+  final SetterFunction setter;
+  StreamSubscription<double> subscription;
+}
+
 abstract class AnimatedComponent extends Component {
 
   AnimatedComponent({ Object key }) : super(key: key, stateful: true);
 
   void syncFields(AnimatedComponent source) { }
 
+  List<_AnimationEntry> _animatedFields = new List<_AnimationEntry>();
+
   animate(AnimatedValue value, SetterFunction setter) {
+    assert(!mounted);
     setter(value.value);
-    StreamSubscription<double> subscription;
-    onDidMount(() {
-      subscription = value.onValueChanged.listen((_) {
-        setter(value.value);
+    _animatedFields.add(new _AnimationEntry(value, setter));
+  }
+
+  void didMount() {
+    for (_AnimationEntry entry in _animatedFields) {
+      entry.subscription = entry.value.onValueChanged.listen((_) {
+        entry.setter(entry.value.value);
         scheduleBuild();
       });
-    });
-    onDidUnmount(() {
-      if (subscription != null) {
-        subscription.cancel();
-        subscription = null;
-      }
-    });
+    }
+    super.didMount();
+  }
+
+  void didUnmount() {
+    for (_AnimationEntry entry in _animatedFields) {
+      assert(entry.subscription != null);
+      entry.subscription.cancel();
+      entry.subscription = null;
+    }
+    super.didUnmount();
   }
 
 }