refactor(testing): remove wrapping of Jasmine functions (#9564)

Instead, the async function now determines whether it should return a promise
or instead call a done function parameter. Importing Jasmine functions
from `@angular/core/testing` is no longer necessary and is now deprecated.

Additionally, beforeEachProviders is also deprecated, as it is specific
to the testing framework. Instead, use the new addProviders method directly.

Before:
```js
import {beforeEachProviders, it, describe, inject} from 'angular2/testing/core';

describe('my code', () => {
  beforeEachProviders(() => [MyService]);

  it('does stuff', inject([MyService], (service) => {
    // actual test
  });
});
```

After:
```js
import {addProviders, inject} from 'angular2/testing/core';

describe('my code', () => {
  beforeEach(() => {
    addProviders([MyService]);
  });

  it('does stuff', inject([MyService], (service) => {
    // actual test
  });
});
```

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

@@ -26,7 +26,7 @@ let _FakeAsyncTestZoneSpecType = (Zone as any /** TODO #9100 */)['FakeAsyncTestZ
  * @param fn
  * @returns {Function} The function wrapped to be executed in the fakeAsync zone
  */
-export function fakeAsync(fn: Function): Function {
+export function fakeAsync(fn: Function): (...args: any[]) => any {
   if (Zone.current.get('FakeAsyncTestZoneSpec') != null) {
     throw new BaseException('fakeAsync() calls can not be nested');
   }