Adding supported checks for getCssCanvasContext.

sdk/lib/html/dart2js/html_dart2js.dart

 library html;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:html_common';
 import 'dart:indexed_db';
 import 'dart:isolate';
 import 'dart:json' as json;
 import 'dart:math';
 import 'dart:svg' as svg;
@@ skipping 10533 matching lines @@
   /** @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]
+   */
+  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);
   }
 
   /** @domName Document.head */
   HeadElement get head => document.$dom_head;
 
   /** @domName Document.lastModified */
   String get lastModified => document.$dom_lastModified;
@@ skipping 16393 matching lines @@
       _position = nextPosition;
       return true;
     }
     _current = null;
     _position = _array.length;
     return false;
   }
 
   T get current => _current;
 }