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/platform-browser/src/dom/dom_tokens.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';
 
 /**
  * A DI Token representing the main rendering context. In a browser this is the DOM Document.
@@ -16,4 +16,4 @@ import {OpaqueToken} from '@angular/core';
  *
  * @stable
  */
-export const DOCUMENT: OpaqueToken = new OpaqueToken('DocumentToken');
+export const DOCUMENT = new InjectionToken<Document>('DocumentToken');

modules/@angular/platform-browser/src/dom/events/event_manager.ts

@@ -6,13 +6,16 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Inject, Injectable, NgZone, OpaqueToken} from '@angular/core';
+import {Inject, Injectable, InjectionToken, NgZone} from '@angular/core';
+
 import {getDOM} from '../dom_adapter';
 
+
 /**
  * @stable
  */
-export const EVENT_MANAGER_PLUGINS: OpaqueToken = new OpaqueToken('EventManagerPlugins');
+export const EVENT_MANAGER_PLUGINS =
+    new InjectionToken<EventManagerPlugin[]>('EventManagerPlugins');
 
 /**
  * @stable

modules/@angular/platform-browser/src/dom/events/hammer_gestures.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Inject, Injectable, OpaqueToken} from '@angular/core';
+import {Inject, Injectable, InjectionToken} from '@angular/core';
 import {EventManagerPlugin} from './event_manager';
 
 const EVENT_NAMES = {
@@ -53,7 +53,7 @@ const EVENT_NAMES = {
  *
  * @experimental
  */
-export const HAMMER_GESTURE_CONFIG: OpaqueToken = new OpaqueToken('HammerGestureConfig');
+export const HAMMER_GESTURE_CONFIG = new InjectionToken<HammerGestureConfig>('HammerGestureConfig');
 
 export interface HammerInstance {
   on(eventName: string, callback: Function): void;