MDL-63667 javascript: fix failed to pre-fetch the template error

lib/amd/src/templates.js

@@ -50,6 +50,9 @@ define(['core/mustache',
     /** @var {Promise[]} templatePromises - Cache of already loaded template promises */
     var templatePromises = {};
 
+    /** @var {Promise[]} cachePartialPromises - Cache of already loaded template partial promises */
+    var cachePartialPromises = {};
+
     /** @var {Object} iconSystem - Object extending core/iconsystem */
     var iconSystem = {};
 
@@ -648,24 +651,26 @@ define(['core/mustache',
      * @return {Promise} JQuery promise object resolved when all partials are in the cache.
      */
     Renderer.prototype.cachePartials = function(templateName) {
-        return this.getTemplate(templateName).then(function(templateSource) {
-            var i;
-            var partials = this.scanForPartials(templateSource);
-            var fetchThemAll = [];
+        var searchKey = this.currentThemeName + '/' + templateName;
 
-            for (i = 0; i < partials.length; i++) {
-                var searchKey = this.currentThemeName + '/' + partials[i];
-                if (searchKey in templatePromises) {
-                    fetchThemAll.push(templatePromises[searchKey]);
-                } else {
-                    fetchThemAll.push(this.cachePartials(partials[i]));
-                }
-            }
+        if (searchKey in cachePartialPromises) {
+            return cachePartialPromises[searchKey];
+        }
+
+        var cachePartialPromise = this.getTemplate(templateName).then(function(templateSource) {
+            var partials = this.scanForPartials(templateSource);
+            var fetchThemAll = partials.map(function(partialName) {
+                return this.cachePartials(partialName);
+            }.bind(this));
 
             return $.when.apply($, fetchThemAll).then(function() {
                 return templateSource;
             });
         }.bind(this));
+
+        cachePartialPromises[searchKey] = cachePartialPromise;
+
+        return cachePartialPromise;
     };
 
     /**