Adding supported checks for WebGL

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 html;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
+  CanvasRenderingContext2D get context2d => getContext('2d');
 
-$if DART2JS
-  CanvasRenderingContext getContext(String contextId) native;
-$endif
-  CanvasRenderingContext2D get context2d => getContext('2d');
+  @SupportedBrowser(SupportedBrowser.CHROME)
+  @SupportedBrowser(SupportedBrowser.FIREFOX)
+  @Experimental
+  WebGLRenderingContext 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;
+  }
 }