Effen: willUmount->didUnmount, allow setState after didUnmount

This patch changes the timing of the unmount call until after the component is removed (and marked as such) and allows setState to be called after this point. If this happens, setState will still invoke the closer to do any neccesary cleanup, but doesn't schedule the component for render

R=eseidel@chromium.org, eseidel
BUG=

Review URL: https://codereview.chromium.org/985853002

examples/fn/widgets/fixedheightscrollable.dart

@@ -64,7 +64,7 @@ abstract class FixedHeightScrollable extends Component {
     ..events.listen('wheel', _handleWheel);
   }
 
-  void willUnmount() {
+  void didUnmount() {
     _stopFling();
   }
 

examples/fn/widgets/material.dart

@@ -61,7 +61,7 @@ abstract class MaterialComponent extends Component {
     });
   }
 
-  void willUnmount() {
+  void didUnmount() {
     _cancelSplashes(null);
   }