fix(testing): async/fakeAsync/inject/withModule helpers should pass through context to callback functions (#13718)

Make sure that context (`this`) that is passed to functions generated by test helpers is passed through to the callback functions. Enables usage of Jasmine's variable sharing system to prevent accidental memory leaks during test runs.

modules/@angular/core/testing/async.ts

@@ -31,14 +31,15 @@ export function async(fn: Function): (done: any) => any {
   // If we're running using the Jasmine test framework, adapt to call the 'done'
   // function when asynchronous activity is finished.
   if (_global.jasmine) {
-    return (done: any) => {
+    // Not using an arrow function to preserve context passed from call site
+    return function(done: any) {
       if (!done) {
         // if we run beforeEach in @angular/core/testing/testing_internal then we get no done
         // fake it here and assume sync.
         done = function() {};
         done.fail = function(e: any) { throw e; };
       }
-      runInTestZone(fn, done, (err: any) => {
+      runInTestZone(fn, this, done, (err: any) => {
         if (typeof err === 'string') {
           return done.fail(new Error(<string>err));
         } else {
@@ -50,12 +51,16 @@ export function async(fn: Function): (done: any) => any {
   // Otherwise, return a promise which will resolve when asynchronous activity
   // is finished. This will be correctly consumed by the Mocha framework with
   // it('...', async(myFn)); or can be used in a custom framework.
-  return () => new Promise<void>((finishCallback, failCallback) => {
-           runInTestZone(fn, finishCallback, failCallback);
-         });
+  // Not using an arrow function to preserve context passed from call site
+  return function() {
+    return new Promise<void>((finishCallback, failCallback) => {
+      runInTestZone(fn, this, finishCallback, failCallback);
+    });
+  };
 }
 
-function runInTestZone(fn: Function, finishCallback: Function, failCallback: Function) {
+function runInTestZone(
+    fn: Function, context: any, finishCallback: Function, failCallback: Function) {
   const currentZone = Zone.current;
   const AsyncTestZoneSpec = (Zone as any)['AsyncTestZoneSpec'];
   if (AsyncTestZoneSpec === undefined) {
@@ -103,5 +108,5 @@ function runInTestZone(fn: Function, finishCallback: Function, failCallback: Fun
         'test');
     proxyZoneSpec.setDelegate(testZoneSpec);
   });
-  return Zone.current.runGuarded(fn);
+  return Zone.current.runGuarded(fn, context);
 }