Add dummy dart:html library for documentation.

lib/html/doc/interface/EmbedElement.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 HTMLEmbedElement
+interface EmbedElement extends Element default _Elements {
+
+  EmbedElement();
+
+  /** @domName HTMLEmbedElement.align */
+  String align;
+
+  /** @domName HTMLEmbedElement.height */
+  String height;
+
+  /** @domName HTMLEmbedElement.name */
+  String name;
+
+  /** @domName HTMLEmbedElement.src */
+  String src;
+
+  /** @domName HTMLEmbedElement.type */
+  String type;
+
+  /** @domName HTMLEmbedElement.width */
+  String width;
+}