`zone.js` 0.8.25 introduces `zone-testing` bundle and move all `fakeAsync/async` logic
from `@angular/core/testing` to `zone.js` package. But in case some user still using the old
version of `zone.js`, an old version of `fakeAsync/async` logic were still kept inside `@angular/core/testing`
package as `fallback` logic. Since now `Angular8+` already use `zone.js 0.9+`, so
those fallback logic is removed.
PR Close#37879
@angular/core/testing provide `async` test utility, but the name `async` is
confusing with the javascript keyword `async`. And in some test case, if you
want to use both the `async` from `@angular/core/testing` and `async/await`,
you may have to write the code like this.
```typescript
it('test async operations', async(async() => {
const result = await asyncMethod();
expect(result).toEqual('expected');
}));
```
So in this PR, the `async` is renamed to `waitForAsync` and also deprecate `async`.
PR Close#37583
