Adding supported checks for getCssCanvasContext.

tools/dom/templates/html/impl/impl_HTMLDocument.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.
 
 // WARNING: Do not edit - generated code.
 
 part of html;
 
 /// @domName $DOMNAME
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
   /** @domName Document.body */
   BodyElement get body => document.$dom_body;
 
   /** @domName Document.body */
   void set body(BodyElement value) {
     document.$dom_body = value;
   }
 
   /** @domName Document.caretRangeFromPoint */
   Range caretRangeFromPoint(int x, int y) {
     return document.$dom_caretRangeFromPoint(x, y);
   }
 
   /** @domName Document.elementFromPoint */
   Element elementFromPoint(int x, int y) {
     return document.$dom_elementFromPoint(x, y);
   }
 
-  /** @domName Document.getCSSCanvasContext */
+  /**
+   * Checks if the getCssCanvasContext API is supported on the current platform.
+   *
+   * See also:
+   *
+   * * [getCssCanvasContext]
+   */
+$if DART2JS
+  static bool get supportsCssCanvasContext =>
+      JS('bool', '!!(document.getCSSCanvasContext)');
+$else
+  static bool get supportsCssCanvasContext => true;
+$endif
+
+
+  /**
+   * Gets a CanvasRenderingContext which can be used as the CSS background of an
+   * element.
+   *
+   * CSS:
+   *
+   *     background: -webkit-canvas(backgroundCanvas)
+   *
+   * Generate the canvas:
+   *
+   *     var context = document.getCssCanvasContext('2d', 'backgroundCanvas',
+   *         100, 100);
+   *     context.fillStyle = 'red';
+   *     context.fillRect(0, 0, 100, 100);
+   *
+   * See also:
+   *
+   * * [supportsCssCanvasContext]
+   * * [CanvasElement.getContext]
+   */
+  @SupportedBrowser(SupportedBrowser.CHROME)
+  @SupportedBrowser(SupportedBrowser.SAFARI)
+  @Experimental()
   CanvasRenderingContext getCssCanvasContext(String contextId, String name,
       int width, int height) {
     return document.$dom_getCssCanvasContext(contextId, name, width, height);
   }
 
   /** @domName Document.head */
   HeadElement get head => document.$dom_head;
 
   /** @domName Document.lastModified */
   String get lastModified => document.$dom_lastModified;
@@ skipping 48 matching lines @@
   /** @domName Document.webkitIsFullScreen */
   bool get webkitIsFullScreen => document.$dom_webkitIsFullScreen;
 
   /** @domName Document.webkitPointerLockElement */
   Element get webkitPointerLockElement =>
       document.$dom_webkitPointerLockElement;
 
   /** @domName Document.webkitVisibilityState */
   String get webkitVisibilityState => document.$dom_webkitVisibilityState;
 }