Add dummy dart:html library for documentation.

lib/html/doc/interface/MediaStreamTrack.dartdoc

Index: lib/html/doc/interface/MediaStreamTrack.dartdoc
diff --git a/lib/html/doc/interface/MediaStreamTrack.dartdoc b/lib/html/doc/interface/MediaStreamTrack.dartdoc
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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.
+
+// WARNING:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName MediaStreamTrack
+interface MediaStreamTrack {
+
+  /** @domName MediaStreamTrack.enabled */
+  bool enabled;
+
+  /** @domName MediaStreamTrack.kind */
+  final String kind;
+
+  /** @domName MediaStreamTrack.label */
+  final String label;
+}