Add dummy dart:html library for documentation.

lib/html/doc/interface/CanvasElement.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.
+
+// WARNING: Do not edit - generated code.
+
+/// @domName HTMLCanvasElement
+interface CanvasElement extends Element default _Elements {
+
+  CanvasElement([int height, int width]);
+
+  /** @domName HTMLCanvasElement.height */
+  int height;
+
+  /** @domName HTMLCanvasElement.width */
+  int width;
+
+  /** @domName HTMLCanvasElement.getContext */
+  Object getContext(String contextId);
+
+  /** @domName HTMLCanvasElement.toDataURL */
+  String toDataURL(String type);
+}