Remove redundant setState() calls around scroll behaviour updates.

We don't need to rebuild when those change; we only got the change because we rebuilt in the first place.

R=jackson@google.com

Review URL: https://codereview.chromium.org/1234863004 .

sdk/lib/widgets/fixed_height_scrollable.dart

@@ -85,6 +85,9 @@ abstract class FixedHeightScrollable extends Scrollable {
     List<Widget> items = buildItems(itemShowIndex, itemShowCount);
     assert(items.every((item) => item.key != null));
 
+    // TODO(ianh): Refactor this so that it does the building in the
+    // same frame as the size observing, similar to BlockViewport, but
+    // keeping the fixed-height optimisations.
     return new SizeObserver(
       callback: _handleSizeChanged,
       child: new Viewport(

sdk/lib/widgets/scrollable_viewport.dart

@@ -23,16 +23,12 @@ class ScrollableViewport extends Scrollable {
   double _viewportHeight = 0.0;
   double _childHeight = 0.0;
   void _handleViewportSizeChanged(Size newSize) {
-    setState(() {
-      _viewportHeight = newSize.height;
-      _updateScrollBehaviour();
-    });
+    _viewportHeight = newSize.height;
+    _updateScrollBehaviour();
   }
   void _handleChildSizeChanged(Size newSize) {
-    setState(() {
-      _childHeight = newSize.height;
-      _updateScrollBehaviour();
-    });
+    _childHeight = newSize.height;
+    _updateScrollBehaviour();
   }
   void _updateScrollBehaviour() {
     scrollBehavior.contentsSize = _childHeight;

sdk/lib/widgets/variable_height_scrollable.dart

@@ -30,16 +30,15 @@ class VariableHeightScrollable extends Scrollable {
   OverscrollBehavior get scrollBehavior => super.scrollBehavior;
 
   void _handleSizeChanged(Size newSize) {
-    setState(() {
-      scrollBehavior.containerSize = newSize.height;
-    });
+    scrollBehavior.containerSize = newSize.height;
   }
 
   void _handleLayoutChanged(
-      int firstVisibleChildIndex,
-      int visibleChildCount,
-      UnmodifiableListView<double> childOffsets,
-      bool didReachLastChild) {
+    int firstVisibleChildIndex,
+    int visibleChildCount,
+    UnmodifiableListView<double> childOffsets,
+    bool didReachLastChild
+  ) {
     assert(childOffsets.length > 0);
     scrollBehavior.contentsSize = didReachLastChild ? childOffsets.last : double.INFINITY;
     if (didReachLastChild && scrollOffset > scrollBehavior.maxScrollOffset)