More precise native type for context2D

tools/dom/templates/html/impl/impl_HTMLCanvasElement.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)class $CLASSNAME$EXTENDS implements CanvasImageSource$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
   /** An API for drawing on this canvas. */
+$if DART2JS
+  CanvasRenderingContext2D get context2D =>
+      JS('Null|CanvasRenderingContext2D', '#.getContext(#)', this, '2d');
+$else
   CanvasRenderingContext2D get context2D => getContext('2d');
+$endif
 
   @deprecated
   CanvasRenderingContext2D get context2d => this.context2D;
 
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.FIREFOX)
   @Experimental
   gl.RenderingContext getContext3d({alpha: true, depth: true, stencil: false,
     antialias: true, premultipliedAlpha: true, preserveDrawingBuffer: false}) {
 
     var options = {
       'alpha': alpha,
       'depth': depth,
       'stencil': stencil,
       'antialias': antialias,
       'premultipliedAlpha': premultipliedAlpha,
       'preserveDrawingBuffer': preserveDrawingBuffer,
     };
     var context = getContext('webgl', options);
     if (context == null) {
       context = getContext('experimental-webgl', options);
     }
     return context;
   }
 }