Add @override annotation to known overriden methods

sky/sdk/lib/painting/box_painter.dart

Index: sky/sdk/lib/painting/box_painter.dart
diff --git a/sky/sdk/lib/painting/box_painter.dart b/sky/sdk/lib/painting/box_painter.dart
index e775a25ac050f1866406581e4331527fe8201140..ea4ff482e97bccda294994c8b0c245b206276d83 100644
--- a/sky/sdk/lib/painting/box_painter.dart
+++ b/sky/sdk/lib/painting/box_painter.dart
@@ -20,12 +20,15 @@ class BorderSide {
 
   static const none = const BorderSide(width: 0.0);
 
+  @override
   int get hashCode {
     int value = 373;
     value = 37 * value * color.hashCode;
     value = 37 * value * width.hashCode;
     return value;
   }
+
+  @override
   String toString() => 'BorderSide($color, $width)';
 }
 
@@ -48,6 +51,7 @@ class Border {
   final BorderSide bottom;
   final BorderSide left;
 
+  @override
   int get hashCode {
     int value = 373;
     value = 37 * value * top.hashCode;
@@ -56,6 +60,8 @@ class Border {
     value = 37 * value * left.hashCode;
     return value;
   }
+
+  @override
   String toString() => 'Border($top, $right, $bottom, $left)';
 }
 
@@ -70,6 +76,7 @@ class BoxShadow {
   final Offset offset;
   final double blur;
 
+  @override
   String toString() => 'BoxShadow($color, $offset, $blur)';
 }
 
@@ -92,9 +99,11 @@ class LinearGradient extends Gradient {
     this.tileMode: sky.TileMode.clamp
   });
 
+  @override
   String toString() =>
       'LinearGradient($endPoints, $colors, $colorStops, $tileMode)';
 
+  @override
   sky.Shader createShader() {
     return new sky.Gradient.linear(this.endPoints, this.colors, this.colorStops,
                                    this.tileMode);
@@ -115,9 +124,11 @@ class RadialGradient extends Gradient {
     this.tileMode: sky.TileMode.clamp
   });
 
+  @override
   String toString() =>
       'RadialGradient($center, $radius, $colors, $colorStops, $tileMode)';
 
+  @override
   sky.Shader createShader() {
     return new sky.Gradient.radial(this.center, this.radius, this.colors,
                                    this.colorStops, this.tileMode);
@@ -173,6 +184,7 @@ class BackgroundImage {
     _listeners.remove(listener);
   }
 
+  @override
   String toString() => 'BackgroundImage($fit, $repeat)';
 }
 
@@ -198,6 +210,7 @@ class BoxDecoration {
   final Gradient gradient;
   final Shape shape;
 
+  @override
   String toString([String prefix = '']) {
     List<String> result = [];
     if (backgroundColor != null)