Adding supported checks for getCssCanvasContext.

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

Index: tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
index e95cc72415f571e491fdd1bc7cdcce88a10333b2..06afcb2631ad4ea996a104ea4ccbca490fcf181e 100644
--- a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
@@ -27,7 +27,44 @@ $!MEMBERS
     return document.$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);