feat(core): Add type information to injector.get() (#13785)

- Introduce `InjectionToken<T>` which is a parameterized and type-safe
  version of `OpaqueToken`.

DEPRECATION:
- `OpaqueToken` is now deprecated, use `InjectionToken<T>` instead.
- `Injector.get(token: any, notFoundValue?: any): any` is now deprecated
  use the same method which is now overloaded as
  `Injector.get<T>(token: Type<T>|InjectionToken<T>, notFoundValue?: T): T;`.

Migration
- Replace `OpaqueToken` with `InjectionToken<?>` and parameterize it.
- Migrate your code to only use `Type<?>` or `InjectionToken<?>` as
  injection tokens. Using other tokens will not be supported in the
  future.

BREAKING CHANGE:
- Because `injector.get()` is now parameterize it is possible that code
  which used to work no longer type checks. Example would be if one
  injects `Foo` but configures it as `{provide: Foo, useClass: MockFoo}`.
  The injection instance will be that of `MockFoo` but the type will be
  `Foo` instead of `any` as in the past. This means that it was possible
  to call a method on `MockFoo` in the past which now will fail type
  check. See this example:

```
class Foo {}
class MockFoo extends Foo {
  setupMock();
}

var PROVIDERS = [
  {provide: Foo, useClass: MockFoo}
];

...

function myTest(injector: Injector) {
  var foo = injector.get(Foo);
  // This line used to work since `foo` used to be `any` before this
  // change, it will now be `Foo`, and `Foo` does not have `setUpMock()`.
  // The fix is to downcast: `injector.get(Foo) as MockFoo`.
  foo.setUpMock();
}
```

PR Close #13785

modules/@angular/forms/src/directives/shared.ts

@@ -116,12 +116,11 @@ function _throwError(dir: AbstractControlDirective, message: string): void {
   throw new Error(`${message} ${messageEnd}`);
 }
 
-export function composeValidators(validators: /* Array<Validator|Function> */ any[]): ValidatorFn {
+export function composeValidators(validators: Array<Validator|Function>): ValidatorFn {
   return isPresent(validators) ? Validators.compose(validators.map(normalizeValidator)) : null;
 }
 
-export function composeAsyncValidators(validators: /* Array<Validator|Function> */ any[]):
-    AsyncValidatorFn {
+export function composeAsyncValidators(validators: Array<Validator|Function>): AsyncValidatorFn {
   return isPresent(validators) ? Validators.composeAsync(validators.map(normalizeAsyncValidator)) :
                                  null;
 }