Typecheck some of ui/webui/resources/js/ with Closure compiler.

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.
  */
 function assert(condition, opt_message) {
   'use strict';
   if (!condition) {
     var msg = 'Assertion failed';
     if (opt_message)
       msg = msg + ': ' + opt_message;
     throw new Error(msg);
   }
 }
+
+/**
+ * Call this from places in the code that should never be reached.
+ *
+ * For example, handling all the values of enum with a switch() like this:
+ *
+ *   function getValueFromEnum(enum) {
+ *     switch (enum) {
+ *       case ENUM_FIRST_OF_TWO:
+ *         return first
+ *       case ENUM_LAST_OF_TWO:
+ *         return last;
+ *     }
+ *     assertNotReached();
+ *     return document;
+ *   }
+ *
+ * This code should only be hit in the case of serious programmer error or
+ * unexpected input.
+ *
+ * @param {string=} opt_message A message to show when this is hit.
+ */
+function assertNotReached(opt_message) {
+  throw new Error(opt_message || "Unreachable code hit");
+}