Add dummy dart:html library for documentation.

lib/html/doc/interface/AudioNode.dartdoc

+// 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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName AudioNode
+interface AudioNode {
+
+  /** @domName AudioNode.context */
+  final AudioContext context;
+
+  /** @domName AudioNode.numberOfInputs */
+  final int numberOfInputs;
+
+  /** @domName AudioNode.numberOfOutputs */
+  final int numberOfOutputs;
+
+  /** @domName AudioNode.connect */
+  void connect(destination, int output, [int input]);
+
+  /** @domName AudioNode.disconnect */
+  void disconnect(int output);
+}