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

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

Index: sky/sdk/lib/framework/components2/scrollable.dart
diff --git a/sky/sdk/lib/framework/components2/scrollable.dart b/sky/sdk/lib/framework/components2/scrollable.dart
index b43583325d934573a60ccf0bc7425b7125fae87c..dee2c137335f2e4647d5b4a3700459085341f169 100644
--- a/sky/sdk/lib/framework/components2/scrollable.dart
+++ b/sky/sdk/lib/framework/components2/scrollable.dart
@@ -23,9 +23,7 @@ abstract class ScrollClient {
 
 abstract class Scrollable extends Component {
 
-  Scrollable({Object key}) : super(key: key, stateful: true) {
-    onDidUnmount(_stopSimulation);
-  }
+  Scrollable({Object key}) : super(key: key, stateful: true);
 
   void syncFields(Scrollable source) { }
 
@@ -102,6 +100,11 @@ abstract class Scrollable extends Component {
     return scrollTo(newScrollOffset);
   }
 
+  void didUnmount() {
+    _stopSimulation();
+    super.didUnmount();
+  }
+
   void _stopSimulation() {
     if (_simulation == null)
       return;