Print Preview: Make getChildElement return required HTMLElement

chrome/browser/resources/print_preview/component.js

 // Copyright (c) 2012 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.
 
 cr.define('print_preview', function() {
   'use strict';
 
   /**
    * Class that represents a UI component.
    * @constructor
@@ skipping 144 matching lines @@
     removeChildren: function() {
       while (this.children_.length > 0) {
         this.removeChild(this.children_[0]);
       }
     },
 
     /**
      * @param {string} query Selector query to select an element starting from
      *     the component's root element using a depth first search for the first
      *     element that matches the query.
-     * @return {HTMLElement} Element selected by the given query.
-     * TODO(alekseys): Check all call sites and rename this function to
-     *     something like getRequiredChildElement.
+     * @return {!HTMLElement} Element selected by the given query.
      */
     getChildElement: function(query) {
-      return this.element_.querySelector(query);
+      return assert(this.element_.querySelector(query));
     },
 
     /**
      * Sets the component's element.
      * @param {Element} element HTML element to set as the component's element.
      * @protected
      */
     setElementInternal: function(element) {
       this.element_ = element;
     },
@@ skipping 22 matching lines @@
         setIsVisible(el, true);
       }
       return el;
     }
   };
 
   return {
     Component: Component
   };
 });