Add dummy dart:html library for documentation.

lib/html/doc/interface/OverflowEvent.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 OverflowEvent
+interface OverflowEvent extends Event {
+
+  static final int BOTH = 2;
+
+  static final int HORIZONTAL = 0;
+
+  static final int VERTICAL = 1;
+
+  /** @domName OverflowEvent.horizontalOverflow */
+  final bool horizontalOverflow;
+
+  /** @domName OverflowEvent.orient */
+  final int orient;
+
+  /** @domName OverflowEvent.verticalOverflow */
+  final bool verticalOverflow;
+}