Add @override annotation to known overriden methods

sky/sdk/lib/widgets/block_viewport.dart

Index: sky/sdk/lib/widgets/block_viewport.dart
diff --git a/sky/sdk/lib/widgets/block_viewport.dart b/sky/sdk/lib/widgets/block_viewport.dart
index 9b5395f798465d4420303ee1a34d43f63d36b722..0c502de1b89b6d5c969130d2b11253a56700ae7f 100644
--- a/sky/sdk/lib/widgets/block_viewport.dart
+++ b/sky/sdk/lib/widgets/block_viewport.dart
@@ -15,7 +15,11 @@ class _Key {
   factory _Key.fromWidget(Widget widget) => new _Key(widget.runtimeType, widget.key);
   final Type type;
   final String key;
+
+  @override
   bool operator ==(other) => other is _Key && other.type == type && other.key == key;
+
+  @override
   int get hashCode => 373 * 37 * type.hashCode + key.hashCode;
 }
 
@@ -27,10 +31,13 @@ class BlockViewport extends RenderObjectWrapper {
   double startOffset;
   Object token;
 
+  @override
   RenderBlockViewport get root => super.root;
+
+  @override
   RenderBlockViewport createNode() => new RenderBlockViewport();
 
-  Map<_Key, Widget> _childrenByKey = new Map<_Key, Widget>();  
+  Map<_Key, Widget> _childrenByKey = new Map<_Key, Widget>();
 
   void walkChildren(WidgetTreeWalker walker) {
     for (Widget child in _childrenByKey.values)
@@ -39,6 +46,7 @@ class BlockViewport extends RenderObjectWrapper {
 
   static const _omit = const Object(); // used as a slot when it's not yet time to attach the child
 
+  @override
   void insertChildRoot(RenderObjectWrapper child, dynamic slot) {
     if (slot == _omit)
       return;
@@ -58,6 +66,7 @@ class BlockViewport extends RenderObjectWrapper {
     assert(root == this.root); // TODO(ianh): Remove this once the analyzer is cleverer
   }
 
+  @override
   void remove() {
     for (Widget child in _childrenByKey.values) {
       assert(child != null);
@@ -66,11 +75,13 @@ class BlockViewport extends RenderObjectWrapper {
     super.remove();
   }
 
+  @override
   void didMount() {
     root.callback = layout;
     super.didMount();
   }
 
+  @override
   void didUnmount() {
     root.callback = null;
     super.didUnmount();
@@ -100,6 +111,7 @@ class BlockViewport extends RenderObjectWrapper {
 
   bool _dirty = false;
 
+  @override
   bool retainStatefulNodeIfPossible(BlockViewport newNode) {
     retainStatefulRenderObjectWrapper(newNode);
     if (startOffset != newNode.startOffset) {
@@ -115,6 +127,7 @@ class BlockViewport extends RenderObjectWrapper {
     return true;
   }
 
+  @override
   void syncRenderObject(BlockViewport old) {
     super.syncRenderObject(old);
     if (_dirty)
@@ -257,7 +270,7 @@ class BlockViewport extends RenderObjectWrapper {
         }
         widget.updateSlot(nextSibling);
         nextSibling = widget.root;
-      }      
+      }
     }
 
     _childrenByKey = newChildren;