Added the CanvasImageSource interface, which all types that a canvas can draw

tools/dom/src/CanvasImageSource.dart

Index: tools/dom/src/CanvasImageSource.dart
diff --git a/tools/dom/src/CanvasImageSource.dart b/tools/dom/src/CanvasImageSource.dart
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+++ b/tools/dom/src/CanvasImageSource.dart
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+// 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].
+ *
+ * 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.
+ */
+abstract class CanvasImageSource {}