Add dummy dart:html library for documentation.

lib/html/doc/interface/MapElement.dart

Index: lib/html/doc/interface/MapElement.dart
diff --git a/lib/html/doc/interface/MapElement.dart b/lib/html/doc/interface/MapElement.dart
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index 0000000000000000000000000000000000000000..a2162660dc503147f0827c43a7ff53ed2d06ddc1
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+++ b/lib/html/doc/interface/MapElement.dart
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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: Do not edit - generated code.
+
+/// @domName HTMLMapElement
+interface MapElement extends Element default _Elements {
+
+  MapElement();
+
+  /** @domName HTMLMapElement.areas */
+  final HTMLCollection areas;
+
+  /** @domName HTMLMapElement.name */
+  String name;
+}