Add dummy dart:html library for documentation.

lib/html/doc/interface/ClientRect.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 ClientRect
+interface ClientRect {
+
+  /** @domName ClientRect.bottom */
+  final num bottom;
+
+  /** @domName ClientRect.height */
+  final num height;
+
+  /** @domName ClientRect.left */
+  final num left;
+
+  /** @domName ClientRect.right */
+  final num right;
+
+  /** @domName ClientRect.top */
+  final num top;
+
+  /** @domName ClientRect.width */
+  final num width;
+}