Adding supported checks for getCssCanvasContext.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index efe10ef2b15256d2be4fab6ac599ad46c005dfe8..2a525136f049e5697bf5105e4fd405cce3036139 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -10551,7 +10551,40 @@ class HtmlDocument extends Document native "*HTMLDocument" {
     return document.$dom_elementFromPoint(x, y);
   }
 
-  /** @domName Document.getCSSCanvasContext */
+  /**
+   * Checks if the getCssCanvasContext API is supported on the current platform.
+   *
+   * See also:
+   *
+   * * [getCssCanvasContext]
+   */
+  static bool get supportsCssCanvasContext =>
+      JS('bool', '!!(document.getCSSCanvasContext)');
+
+
+  /**
+   * 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);