--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -139,17 +139,16 @@ function Tester(aTests, aDumper, aCallba
         /*known*/tolerate,
         /*stack*/stack));
     }.bind(this);
 }
 Tester.prototype = {
   EventUtils: {},
   SimpleTest: {},
   Task: null,
-  Promise: null,
   Assert: null,
 
   repeat: 0,
   runUntilFailure: false,
   checker: null,
   currentTestIndex: -1,
   lastStartTime: null,
   openedWindows: null,
@@ -582,17 +581,16 @@ Tester.prototype = {
     let currentScope = this.currentTest.scope = new testScope(this, this.currentTest, this.currentTest.expected);
     let currentTest = this.currentTest;
 
     // Import utils in the test scope.
     this.currentTest.scope.EventUtils = this.EventUtils;
     this.currentTest.scope.SimpleTest = this.SimpleTest;
     this.currentTest.scope.gTestPath = this.currentTest.path;
     this.currentTest.scope.Task = this.Task;
-    this.currentTest.scope.Promise = this.Promise;
     // Pass a custom report function for mochitest style reporting.
     this.currentTest.scope.Assert = new this.Assert(function(err, message, stack) {
       let res;
       if (err) {
         res = new testResult(false, err.message, err.stack, false, err.stack);
       } else {
         res = new testResult(true, message, "", false, stack);
       }
@@ -638,30 +636,31 @@ Tester.prototype = {
       this.Promise.Debugging.flushUncaughtErrors();
       // Run the test
       this.lastStartTime = Date.now();
       if (this.currentTest.scope.__tasks) {
         // This test consists of tasks, added via the `add_task()` API.
         if ("test" in this.currentTest.scope) {
           throw "Cannot run both a add_task test and a normal test at the same time.";
         }
-        this.Task.spawn(function() {
+        let Promise = this.Promise;
+        this.Task.spawn(function*() {
           let task;
           while ((task = this.__tasks.shift())) {
             this.SimpleTest.info("Entering test " + task.name);
             try {
               yield task();
             } catch (ex) {
               let isExpected = !!this.SimpleTest.isExpectingUncaughtException();
               let stack = (typeof ex == "object" && "stack" in ex)?ex.stack:null;
               let name = "Uncaught exception";
               let result = new testResult(isExpected, name, ex, false, stack);
               currentTest.addResult(result);
             }
-            this.Promise.Debugging.flushUncaughtErrors();
+            Promise.Debugging.flushUncaughtErrors();
             this.SimpleTest.info("Leaving test " + task.name);
           }
           this.finish();
         }.bind(currentScope));
       } else if ("generatorTest" in this.currentTest.scope) {
         if ("test" in this.currentTest.scope) {
           throw "Cannot run both a generator test and a normal test at the same time.";
         }
@@ -976,17 +975,16 @@ testScope.prototype = {
   __timeoutFactor: 1,
   __expectedMinAsserts: 0,
   __expectedMaxAsserts: 0,
   __expected: 'pass',
 
   EventUtils: {},
   SimpleTest: {},
   Task: null,
-  Promise: null,
   Assert: null,
 
   /**
    * Add a test function which is a Task function.
    *
    * Task functions are functions fed into Task.jsm's Task.spawn(). They are
    * generators that emit promises.
    *