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/common/src/location/location_strategy.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {OpaqueToken} from '@angular/core';
+import {InjectionToken} from '@angular/core';
 import {LocationChangeListener} from './platform_location';
 
 /**
@@ -61,4 +61,4 @@ export abstract class LocationStrategy {
  *
  * @stable
  */
-export const APP_BASE_HREF: OpaqueToken = new OpaqueToken('appBaseHref');
+export const APP_BASE_HREF = new InjectionToken<string>('appBaseHref');

modules/@angular/common/test/directives/ng_component_outlet_spec.ts

@@ -8,7 +8,7 @@
 
 import {CommonModule} from '@angular/common';
 import {NgComponentOutlet} from '@angular/common/src/directives/ng_component_outlet';
-import {Component, ComponentRef, Inject, Injector, NO_ERRORS_SCHEMA, NgModule, OpaqueToken, Optional, Provider, QueryList, ReflectiveInjector, TemplateRef, Type, ViewChild, ViewChildren, ViewContainerRef} from '@angular/core';
+import {Component, ComponentRef, Inject, InjectionToken, Injector, NO_ERRORS_SCHEMA, NgModule, Optional, Provider, QueryList, ReflectiveInjector, TemplateRef, Type, ViewChild, ViewChildren, ViewContainerRef} from '@angular/core';
 import {TestBed, async} from '@angular/core/testing';
 import {expect} from '@angular/platform-browser/testing/matchers';
 
@@ -146,7 +146,7 @@ export function main() {
   });
 }
 
-const TEST_TOKEN = new OpaqueToken('TestToken');
+const TEST_TOKEN = new InjectionToken('TestToken');
 @Component({selector: 'injected-component', template: 'foo'})
 class InjectedComponent {
   constructor(@Optional() @Inject(TEST_TOKEN) public testToken: any) {}