feat(core): add support for using async/await with Jasmine (#24637)

Example:
```
it('...', await(async() => {
  doSomething();
  await asyncFn();
  doSomethingAfter();
}));
```

PR Close #24637

packages/core/test/testability/jasmine_await_spec.ts

@@ -0,0 +1,31 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {jasmineAwait} from '../../testing';
+
+
+describe('await', () => {
+  let pass: boolean;
+  beforeEach(() => pass = false);
+  afterEach(() => expect(pass).toBe(true));
+
+  it('should convert passes', jasmineAwait(async() => { pass = await Promise.resolve(true); }));
+
+  it('should convert failures', (done) => {
+    const error = new Error();
+    const fakeDone: DoneFn = function() { fail('passed, but should have failed'); } as any;
+    fakeDone.fail = function(value: any) {
+      expect(value).toBe(error);
+      done();
+    };
+    jasmineAwait(async() => {
+      pass = await Promise.resolve(true);
+      throw error;
+    })(fakeDone);
+  });
+});

packages/core/testing/src/jasmine_await.ts

@@ -0,0 +1,34 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+/**
+ * Converts an `async` function, with `await`, into a function which is compatible with Jasmine test
+ * framework.
+ *
+ * For asynchronous function blocks, Jasmine expects `it` (and friends) to take a function which
+ * takes a `done` callback. (Jasmine does not understand functions which return `Promise`.) The
+ * `jasmineAwait()` wrapper converts the test function returning `Promise` into a function which
+ * Jasmine understands.
+ *
+ *
+ * Example:
+ * ```
+ * it('...', jasmineAwait(async() => {
+ *   doSomething();
+ *   await asyncFn();
+ *   doSomethingAfter();
+ * }));
+ * ```
+ *
+ */
+export function jasmineAwait(fn: () => Promise<any>):
+    (done: {(): void; fail: (message?: Error | string) => void;}) => void {
+  return function(done: {(): void; fail: (message?: Error | string) => void;}) {
+    fn().then(done, done.fail);
+  };
+}

packages/core/testing/src/testing.ts

@@ -13,6 +13,7 @@
  */
 
 export * from './async';
+export * from './jasmine_await';
 export * from './component_fixture';
 export * from './fake_async';
 export * from './test_bed';

tools/public_api_guard/core/testing.d.ts

@@ -53,6 +53,11 @@ export declare class InjectSetupWrapper {
     inject(tokens: any[], fn: Function): () => any;
 }
 
+export declare function jasmineAwait(fn: () => Promise<any>): (done: {
+    (): void;
+    fail: (message?: Error | string) => void;
+}) => void;
+
 /** @experimental */
 export declare type MetadataOverride<T> = {
     add?: Partial<T>;