Rename @Injectable({scope -> providedIn}).
Instead of {providedIn: APP_ROOT_SCOPE}, accept {providedIn: 'root'}.
Also, {providedIn: null} implies the injectable should not be added
to any scope.
PR Close#22655
Previously the injectable compiler assumed all tree-shakeable injectables
would have dependencies that were injectables or InjectionTokens. However
old code still uses string tokens (e.g. NgUpgrade and '$injector'). Using
such tokens would cause the injectable compiler to crash.
Now, the injectable compiler can properly generate a dependency on such a
string token.
PR Close#22376
