Typecheck JS files for chrome://help before doing import transition

third_party/closure_compiler/externs/pending_compiler_externs.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview Temporary externs until compiler/compiler.jar catches up.
  * @externs
  */
 
 /**
  * @see http://dom.spec.whatwg.org/#dom-domimplementation-createhtmldocument
  * @param {string=} opt_title The title to give the new HTML document.
  * @return {!HTMLDocument}
  */
 DOMImplementation.prototype.createHTMLDocument = function(opt_title) {};
+
+/**
+ * @see http://www.whatwg.org/specs/web-apps/current-work/#popstateevent
+ * @constructor
+ * @extends {Event}
+ *
+ * @param {string} type
+ * @param {{state: *}=} opt_eventInitDict
+ */
+function PopStateEvent(type, opt_eventInitDict) {}