Migrate to Chrome 37 IDL

tools/dom/templates/html/impl/impl_CanvasRenderingContext2D.darttemplate

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of $LIBRARYNAME;
 
-$(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
+abstract class CanvasRenderingContext {
+  CanvasElement get canvas;
+}
+
+$(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS implements CanvasRenderingContext {
 $!MEMBERS
 
   /**
    * Sets the color used inside shapes.
    * [r], [g], [b] are 0-255, [a] is 0-1.
    */
   void setFillColorRgb(int r, int g, int b, [num a = 1]) {
     this.fillStyle = 'rgba($r, $g, $b, $a)';
   }
 
@@ skipping 332 matching lines @@
     JS('void', '#.fill(#)', this, winding);
   }
 $endif
 
   /** Deprecated always returns 1.0 */
   @DomName('CanvasRenderingContext2D.webkitBackingStorePixelRation')
   @Experimental()
   @deprecated
   double get backingStorePixelRatio => 1.0;
 }