Add dummy dart:html library for documentation.

lib/html/doc/interface/HTMLAllCollection.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 HTMLAllCollection
+interface HTMLAllCollection {
+
+  /** @domName HTMLAllCollection.length */
+  final int length;
+
+  /** @domName HTMLAllCollection.item */
+  Node item(int index);
+
+  /** @domName HTMLAllCollection.namedItem */
+  Node namedItem(String name);
+
+  /** @domName HTMLAllCollection.tags */
+  NodeList tags(String name);
+}