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

ui/webui/resources/js/assert.js

 // Copyright (c) 2013 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 Assertion support.
  */
 
 /**
- * Simple common assertion API
- * @param {*} condition The condition to test.  Note that this may be used to
- *     test whether a value is defined or not, and we don't want to force a
- *     cast to Boolean.
- * @param {string=} opt_message A message to use in any error.
+ * Verify |condition| is truthy and return |condition| if so.
+ * @template {T}
+ * @param {T} condition A condition to check for truthiness.  Note that this
+ *     may be used to test whether a value is defined or not, and we don't want
+ *     to force a cast to Boolean.
+ * @param {string=} opt_message A message to show on failure.
+ * @return {!T} A non-null |condition|.
  */
 function assert(condition, opt_message) {
   'use strict';
   if (!condition) {
     var msg = 'Assertion failed';
     if (opt_message)
       msg = msg + ': ' + opt_message;
     throw new Error(msg);
   }
+  return condition;
 }
 
 /**
  * Insert a notReached() in cases where control flow should never hit.
  * @param {string=} opt_message A message to show when this is hit.
  */
 function notReached(opt_message) {
   throw new Error(opt_message || "Unreachable code hit");
 }