Update drawImage to create more fast paths.

tools/dom/src/CanvasImageSource.dart

 // 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;
 
 /**
  * An object that can be drawn to a [CanvasRenderingContext2D] object with
- * [CanvasRenderingContext2D.drawImage] or
- * [CanvasRenderingContext2D.drawImageAtScale].
+ * [CanvasRenderingContext2D.drawImage],
+ * [CanvasRenderingContext2D.drawImageRect],
+ * [CanvasRenderingContext2D.drawImageScaled], or
+ * [CanvasRenderingContext2D.drawImageScaledFromSource].
  *
  * If the CanvasImageSource is an [ImageElement] then the element's image is
  * used. If the [ImageElement] is an animated image, then the poster frame is
  * used. If there is no poster frame, then the first frame of animation is used.
  *
  * If the CanvasImageSource is a [VideoElement] then the frame at the current
  * playback position is used as the image.
  *
  * If the CanvasImageSource is a [CanvasElement] then the element's bitmap is
  * used.
  *
  * See also:
  *
  *  * [CanvasImageSource](http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#image-sources-for-2d-rendering-contexts)
  * from the WHATWG.
+ *  * [drawImage](http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-drawimage)
+ * from the WHATWG.
  */
 abstract class CanvasImageSource {}