MDL-54915 core: JS Blocks must not used shared variables

The JS blocks in a template must _not_ use a shared variable as they can be
executed asynchronously and complete in any order.

Instead it is stored in the only local variable available - the current
context.

Since the context can be passed in from elsewhere, we namespace it to
reduce the chance of collission.

lib/amd/src/templates.js

@@ -45,9 +45,6 @@ define([ 'core/mustache',
     /** @var {string[]} requiredStrings - Collection of strings found during the rendering of one template */
     var requiredStrings = [];
 
-    /** @var {string[]} requiredJS - Collection of js blocks found during the rendering of one template */
-    var requiredJS = [];
-
     /** @var {Number} uniqid Incrementing value that is changed for every call to render */
     var uniqid = 1;
 
@@ -181,7 +178,7 @@ define([ 'core/mustache',
      * @return {string}
      */
     var jsHelper = function(sectionText, helper) {
-        requiredJS.push(helper(sectionText, this));
+        this.jsBlocks.push(helper(sectionText, this));
         return '';
     };
 
@@ -257,11 +254,11 @@ define([ 'core/mustache',
     var addHelpers = function(context, themeName) {
         currentThemeName = themeName;
         requiredStrings = [];
-        requiredJS = [];
         context.uniqid = uniqid++;
         context.str = function() { return stringHelper; };
         context.pix = function() { return pixHelper; };
         context.js = function() { return jsHelper; };
+        context.jsBlocks = [];
         context.quote = function() { return quoteHelper; };
         context.globals = { config : config };
         context.currentTheme = themeName;
@@ -277,8 +274,8 @@ define([ 'core/mustache',
      */
     var getJS = function(strings) {
         var js = '';
-        if (requiredJS.length > 0) {
-            js = requiredJS.join(";\n");
+        if (this.jsBlocks.length > 0) {
+            js = this.jsBlocks.join(";\n");
         }
 
         // Re-render to get the final strings.
@@ -394,11 +391,11 @@ define([ 'core/mustache',
                         // and cause the template to die on the second pass (unbalanced).
 
                         result = treatStringsInContent(result, strings);
-                        deferred.resolve(result, getJS(strings));
+                        deferred.resolve(result, getJS.bind(context)(strings));
                     })
                     .fail(deferred.reject);
                 } else {
-                    deferred.resolve(result.trim(), getJS([]));
+                    deferred.resolve(result.trim(), getJS.bind(context)([]));
                 }
             }
         ).fail(deferred.reject);