Add dummy dart:html library for documentation.

lib/html/doc/interface/ScriptElement.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 HTMLScriptElement
+interface ScriptElement extends Element default _Elements {
+
+  ScriptElement();
+
+  /** @domName HTMLScriptElement.async */
+  bool async;
+
+  /** @domName HTMLScriptElement.charset */
+  String charset;
+
+  /** @domName HTMLScriptElement.crossOrigin */
+  String crossOrigin;
+
+  /** @domName HTMLScriptElement.defer */
+  bool defer;
+
+  /** @domName HTMLScriptElement.event */
+  String event;
+
+  /** @domName HTMLScriptElement.htmlFor */
+  String htmlFor;
+
+  /** @domName HTMLScriptElement.src */
+  String src;
+
+  /** @domName HTMLScriptElement.type */
+  String type;
+}