Add dummy dart:html library for documentation.

lib/html/doc/interface/ProcessingInstruction.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 ProcessingInstruction
+interface ProcessingInstruction extends Node {
+
+  /** @domName ProcessingInstruction.data */
+  String data;
+
+  /** @domName ProcessingInstruction.sheet */
+  final StyleSheet sheet;
+
+  /** @domName ProcessingInstruction.target */
+  final String target;
+}