Add @override annotation to known overriden methods

sky/sdk/lib/widgets/scaffold.dart

Index: sky/sdk/lib/widgets/scaffold.dart
diff --git a/sky/sdk/lib/widgets/scaffold.dart b/sky/sdk/lib/widgets/scaffold.dart
index 3a50fab54f02fa0b8dd084dbf36604e918e1b955..17f5dd33476f15979e153938254c0a125e3b909e 100644
--- a/sky/sdk/lib/widgets/scaffold.dart
+++ b/sky/sdk/lib/widgets/scaffold.dart
@@ -54,6 +54,7 @@ class RenderScaffold extends RenderBox {
     markNeedsLayout();
   }
 
+  @override
   void attachChildren() {
     for (ScaffoldSlots slot in ScaffoldSlots.values) {
       RenderBox box = _slots[slot];
@@ -62,6 +63,7 @@ class RenderScaffold extends RenderBox {
     }
   }
 
+  @override
   void detachChildren() {
     for (ScaffoldSlots slot in ScaffoldSlots.values) {
       RenderBox box = _slots[slot];
@@ -81,7 +83,10 @@ class RenderScaffold extends RenderBox {
     return null;
   }
 
+  @override
   bool get sizedByParent => true;
+
+  @override
   void performResize() {
     size = constraints.biggest;
     assert(!size.isInfinite);
@@ -92,6 +97,7 @@ class RenderScaffold extends RenderBox {
   static const kButtonX = 16.0; // left from right edge of body
   static const kButtonY = 16.0; // up from bottom edge of body
 
+  @override
   void performLayout() {
     double bodyHeight = size.height;
     double bodyPosition = 0.0;
@@ -142,6 +148,7 @@ class RenderScaffold extends RenderBox {
     }
   }
 
+  @override
   void paint(PaintingCanvas canvas, Offset offset) {
     for (ScaffoldSlots slot in ScaffoldSlots.values) {
       RenderBox box = _slots[slot];
@@ -152,6 +159,7 @@ class RenderScaffold extends RenderBox {
     }
   }
 
+  @override
   void hitTestChildren(HitTestResult result, { Point position }) {
     for (ScaffoldSlots slot in ScaffoldSlots.values.reversed) {
       RenderBox box = _slots[slot];
@@ -165,6 +173,7 @@ class RenderScaffold extends RenderBox {
     }
   }
 
+  @override
   String debugDescribeChildren(String prefix) {
     return _slots.keys.map((slot) => '${prefix}${slot}: ${_slots[slot].toString(prefix)}').join();
   }
@@ -191,9 +200,13 @@ class Scaffold extends RenderObjectWrapper {
 
   Map<ScaffoldSlots, Widget> _slots = new Map<ScaffoldSlots, Widget>();
 
+  @override
   RenderScaffold get root => super.root;
+
+  @override
   RenderScaffold createNode() => new RenderScaffold();
 
+  @override
   void walkChildren(WidgetTreeWalker walker) {
     for (ScaffoldSlots slot in ScaffoldSlots.values) {
       Widget widget = _slots[slot];
@@ -202,10 +215,12 @@ class Scaffold extends RenderObjectWrapper {
     }
   }
 
+  @override
   void insertChildRoot(RenderObjectWrapper child, ScaffoldSlots slot) {
     root[slot] = child != null ? child.root : null;
   }
 
+  @override
   void detachChildRoot(RenderObjectWrapper child) {
     final root = this.root; // TODO(ianh): Remove this once the analyzer is cleverer
     assert(root is RenderScaffold);
@@ -214,11 +229,13 @@ class Scaffold extends RenderObjectWrapper {
     assert(root == this.root); // TODO(ianh): Remove this once the analyzer is cleverer
   }
 
+  @override
   void remove() {
     walkChildren((Widget child) => removeChild(child));
     super.remove();
   }
 
+  @override
   void syncRenderObject(Widget old) {
     super.syncRenderObject(old);
     for (ScaffoldSlots slot in ScaffoldSlots.values) {