`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
JiaLiPassion
Joey Perrott
Danny Skoog
Paul Gschwendtner