Add dummy dart:html library for documentation.

lib/html/doc/interface/AudioBuffer.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 AudioBuffer
+interface AudioBuffer {
+
+  /** @domName AudioBuffer.duration */
+  final num duration;
+
+  /** @domName AudioBuffer.gain */
+  num gain;
+
+  /** @domName AudioBuffer.length */
+  final int length;
+
+  /** @domName AudioBuffer.numberOfChannels */
+  final int numberOfChannels;
+
+  /** @domName AudioBuffer.sampleRate */
+  final num sampleRate;
+
+  /** @domName AudioBuffer.getChannelData */
+  Float32Array getChannelData(int channelIndex);
+}