Add @override annotation to known overriden methods

sky/sdk/lib/rendering/stack.dart

Index: sky/sdk/lib/rendering/stack.dart
diff --git a/sky/sdk/lib/rendering/stack.dart b/sky/sdk/lib/rendering/stack.dart
index c28a59f93e9388e19e32276710a66343de5e854f..3cd32395722ff223b70832bab7395cbdd2bc916b 100644
--- a/sky/sdk/lib/rendering/stack.dart
+++ b/sky/sdk/lib/rendering/stack.dart
@@ -13,6 +13,7 @@ class StackParentData extends BoxParentData with ContainerParentDataMixin<Render
   double bottom;
   double left;
 
+  @override
   void merge(StackParentData other) {
     if (other.top != null)
       top = other.top;
@@ -27,6 +28,7 @@ class StackParentData extends BoxParentData with ContainerParentDataMixin<Render
 
   bool get isPositioned => top != null || right != null || bottom != null || left != null;
 
+  @override
   String toString() => '${super.toString()}; top=$top; right=$right; bottom=$bottom, left=$left';
 }
 
@@ -38,12 +40,13 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
     addAll(children);
   }
 
+  @override
   void setupParentData(RenderBox child) {
     if (child.parentData is! StackParentData)
       child.parentData = new StackParentData();
   }
 
-
+  @override
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     double width = constraints.minWidth;
     RenderBox child = firstChild;
@@ -57,6 +60,7 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
     return width;
   }
 
+  @override
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
     bool hasNonPositionedChildren = false;
     double width = constraints.minWidth;
@@ -75,6 +79,7 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
     return width;
   }
 
+  @override
   double getMinIntrinsicHeight(BoxConstraints constraints) {
     double height = constraints.minHeight;
     RenderBox child = firstChild;
@@ -88,6 +93,7 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
     return height;
   }
 
+  @override
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
     bool hasNonPositionedChildren = false;
     double height = constraints.minHeight;
@@ -106,10 +112,12 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
     return height;
   }
 
+  @override
   double computeDistanceToActualBaseline(TextBaseline baseline) {
     return defaultComputeDistanceToHighestActualBaseline(baseline);
   }
 
+  @override
   void performLayout() {
     bool hasNonPositionedChildren = false;
 
@@ -191,10 +199,12 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
     }
   }
 
+  @override
   void hitTestChildren(HitTestResult result, { Point position }) {
     defaultHitTestChildren(result, position: position);
   }
 
+  @override
   void paint(PaintingCanvas canvas, Offset offset) {
     defaultPaint(canvas, offset);
   }