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/platform-browser/test/testing_public_spec.ts

@@ -114,31 +114,63 @@ class CompWithUrlTemplate {
 
 export function main() {
   describe('public testing API', () => {
-    describe('using the async helper', () => {
-      let actuallyDone: boolean;
+    describe('using the async helper with context passing', () => {
+      beforeEach(function() { this.actuallyDone = false; });
 
-      beforeEach(() => actuallyDone = false);
+      afterEach(function() { expect(this.actuallyDone).toEqual(true); });
 
-      afterEach(() => expect(actuallyDone).toEqual(true));
+      it('should run normal tests', function() { this.actuallyDone = true; });
 
-      it('should run normal tests', () => actuallyDone = true);
-
-      it('should run normal async tests', (done) => {
+      it('should run normal async tests', function(done) {
         setTimeout(() => {
-          actuallyDone = true;
+          this.actuallyDone = true;
           done();
         }, 0);
       });
 
       it('should run async tests with tasks',
-         async(() => setTimeout(() => actuallyDone = true, 0)));
+         async(function() { setTimeout(() => this.actuallyDone = true, 0); }));
 
-      it('should run async tests with promises', async(() => {
+      it('should run async tests with promises', async(function() {
            const p = new Promise((resolve, reject) => setTimeout(resolve, 10));
-           p.then(() => actuallyDone = true);
+           p.then(() => this.actuallyDone = true);
          }));
     });
 
+    describe('basic context passing to inject, fakeAsync and withModule helpers', () => {
+      const moduleConfig = {
+        providers: [FancyService],
+      };
+
+      beforeEach(function() { this.contextModified = false; });
+
+      afterEach(function() { expect(this.contextModified).toEqual(true); });
+
+      it('should pass context to inject helper',
+         inject([], function() { this.contextModified = true; }));
+
+      it('should pass context to fakeAsync helper',
+         fakeAsync(function() { this.contextModified = true; }));
+
+      it('should pass context to withModule helper - simple',
+         withModule(moduleConfig, function() { this.contextModified = true; }));
+
+      it('should pass context to withModule helper - advanced',
+         withModule(moduleConfig).inject([FancyService], function(service: FancyService) {
+           expect(service.value).toBe('real value');
+           this.contextModified = true;
+         }));
+
+      it('should preserve context when async and inject helpers are combined',
+         async(inject([], function() { setTimeout(() => this.contextModified = true, 0); })));
+
+      it('should preserve context when fakeAsync and inject helpers are combined',
+         fakeAsync(inject([], function() {
+           setTimeout(() => this.contextModified = true, 0);
+           tick(1);
+         })));
+    });
+
     describe('using the test injector with the inject helper', () => {
       describe('setting up Providers', () => {
         beforeEach(() => {