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/test_bed.ts

@@ -325,10 +325,10 @@ export class TestBed implements Injector {
     return result === UNDEFINED ? this._compiler.injector.get(token, notFoundValue) : result;
   }
 
-  execute(tokens: any[], fn: Function): any {
+  execute(tokens: any[], fn: Function, context?: any): any {
     this._initIfNeeded();
     const params = tokens.map(t => this.get(t));
-    return fn(...params);
+    return fn.apply(context, params);
   }
 
   overrideModule(ngModule: Type<any>, override: MetadataOverride<NgModule>): void {
@@ -413,17 +413,19 @@ export function getTestBed() {
 export function inject(tokens: any[], fn: Function): () => any {
   const testBed = getTestBed();
   if (tokens.indexOf(AsyncTestCompleter) >= 0) {
-    return () =>
-               // Return an async test method that returns a Promise if AsyncTestCompleter is one of
-        // the
-        // injected tokens.
-        testBed.compileComponents().then(() => {
-          const completer: AsyncTestCompleter = testBed.get(AsyncTestCompleter);
-          testBed.execute(tokens, fn);
-          return completer.promise;
-        });
+    // Not using an arrow function to preserve context passed from call site
+    return function() {
+      // Return an async test method that returns a Promise if AsyncTestCompleter is one of
+      // the injected tokens.
+      return testBed.compileComponents().then(() => {
+        const completer: AsyncTestCompleter = testBed.get(AsyncTestCompleter);
+        testBed.execute(tokens, fn, this);
+        return completer.promise;
+      });
+    };
   } else {
-    return () => testBed.execute(tokens, fn);
+    // Not using an arrow function to preserve context passed from call site
+    return function() { return testBed.execute(tokens, fn, this); };
   }
 }
 
@@ -441,9 +443,11 @@ export class InjectSetupWrapper {
   }
 
   inject(tokens: any[], fn: Function): () => any {
-    return () => {
-      this._addModule();
-      return inject(tokens, fn)();
+    const self = this;
+    // Not using an arrow function to preserve context passed from call site
+    return function() {
+      self._addModule();
+      return inject(tokens, fn).call(this);
     };
   }
 }
@@ -456,12 +460,13 @@ export function withModule(moduleDef: TestModuleMetadata, fn: Function): () => a
 export function withModule(moduleDef: TestModuleMetadata, fn: Function = null): (() => any)|
     InjectSetupWrapper {
   if (fn) {
-    return () => {
+    // Not using an arrow function to preserve context passed from call site
+    return function() {
       const testBed = getTestBed();
       if (moduleDef) {
         testBed.configureTestingModule(moduleDef);
       }
-      return fn();
+      return fn.apply(this);
     };
   }
   return new InjectSetupWrapper(() => moduleDef);