Ensure existence of queried elements.

ui/webui/resources/js/util.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.
 
 <include src="assert.js">
 
 /**
  * Alias for document.getElementById.
  * @param {string} id The ID of the element to find.
  * @return {HTMLElement} The found element or null if not found.
@@ skipping 243 matching lines @@
  * calling getElementById and not checking the result because this lets us
  * satisfy the JSCompiler type system.
  * @param {string} id The identifier name.
  * @return {!HTMLElement} the Element.
  */
 function getRequiredElement(id) {
   return assertInstanceof($(id), HTMLElement,
                           'Missing required element: ' + id);
 }
 
+/**
+ * Query an element that's known to exist by a selector. We use this instead of
+ * just calling querySelector and not checking the result because this lets us
+ * satisfy the JSCompiler type system.
+ * @param {(!Document|!DocumentFragment|!Element)} context The context object
+ *     for querySelector.
+ * @param {string} selectors CSS selectors to query the element.
+ * @return {!HTMLElement} the Element.
+ */
+function queryRequiredElement(context, selectors) {
+  var element = context.querySelector(selectors);
+  return assertInstanceof(element, HTMLElement,
+                          'Missing required element: ' + selectors);
+}
+
 // Handle click on a link. If the link points to a chrome: or file: url, then
 // call into the browser to do the navigation.
 document.addEventListener('click', function(e) {
   if (e.defaultPrevented)
     return;
 
   var el = e.target;
   if (el.nodeType == Node.ELEMENT_NODE &&
       el.webkitMatchesSelector('A, A *')) {
     while (el.tagName != 'A') {
@@ skipping 157 matching lines @@
  * @param {number} maxLength The maximum length allowed for the string.
  * @return {string} The original string if its length does not exceed
  *     |maxLength|. Otherwise the first |maxLength| - 1 characters with '...'
  *     appended.
  */
 function elide(original, maxLength) {
   if (original.length <= maxLength)
     return original;
   return original.substring(0, maxLength - 1) + '\u2026';
 }