refactor: `EventEmitter` to retain behaviour of pre TypeScript 3.9 (#36989)

TypeScript 3.9 introduced a breaking change where extends `any` no longer acts as `any`, instead it acts as `unknown`.

With this change we retain the behavior we had with TS 3.8 which is;

When using the `EventEmitter` as a type you must always provide a  type;
```ts
let emitter: EventEmitter<string>
```

and when initializing the `EventEmitter` class you can either provide a  type or or use the fallback type which is `any`

```ts
const emitter = new EventEmitter(); // EventEmitter<any>
const emitter = new EventEmitte<string>(); // EventEmitter<string>
``

PR Close #36989

goldens/public-api/core/core.d.ts

@@ -313,12 +313,18 @@ export declare class ErrorHandler {
     handleError(error: any): void;
 }
 
-export declare class EventEmitter<T> extends Subject<T> {
-    constructor(isAsync?: boolean);
+export declare interface EventEmitter<T> extends Subject<T> {
+    new (isAsync?: boolean): EventEmitter<T>;
     emit(value?: T): void;
     subscribe(generatorOrNext?: any, error?: any, complete?: any): Subscription;
 }
 
+export declare const EventEmitter: {
+    new (isAsync?: boolean): EventEmitter<any>;
+    new <T>(isAsync?: boolean): EventEmitter<T>;
+    readonly prototype: EventEmitter<any>;
+};
+
 export declare interface ExistingProvider extends ExistingSansProvider {
     multi?: boolean;
     provide: any;

packages/core/src/event_emitter.ts

@@ -61,32 +61,26 @@ import {Subject, Subscription} from 'rxjs';
  * @see [Observables in Angular](guide/observables-in-angular)
  * @publicApi
  */
-export class EventEmitter<T> extends Subject<T> {
+export interface EventEmitter<T> extends Subject<T> {
   /**
    * @internal
    */
-  __isAsync: boolean;  // tslint:disable-line
+  __isAsync: boolean;
 
   /**
    * Creates an instance of this class that can
    * deliver events synchronously or asynchronously.
    *
-   * @param isAsync When true, deliver events asynchronously.
+   * @param [isAsync=false] When true, deliver events asynchronously.
    *
    */
-  constructor(isAsync: boolean = false) {
-    super();
-    this.__isAsync = isAsync;
-  }
+  new(isAsync?: boolean): EventEmitter<T>;
 
   /**
    * Emits an event containing a given value.
    * @param value The value to emit.
    */
-  emit(value?: T) {
-    super.next(value);
-  }
-
+  emit(value?: T): void;
   /**
    * Registers handlers for events emitted by this instance.
    * @param generatorOrNext When supplied, a custom handler for emitted events.
@@ -95,6 +89,21 @@ export class EventEmitter<T> extends Subject<T> {
    * @param complete When supplied, a custom handler for a completion
    * notification from this emitter.
    */
+  subscribe(generatorOrNext?: any, error?: any, complete?: any): Subscription;
+}
+
+class EventEmitter_ extends Subject<any> {
+  __isAsync: boolean;  // tslint:disable-line
+
+  constructor(isAsync: boolean = false) {
+    super();
+    this.__isAsync = isAsync;
+  }
+
+  emit(value?: any) {
+    super.next(value);
+  }
+
   subscribe(generatorOrNext?: any, error?: any, complete?: any): Subscription {
     let schedulerFn: (t: any) => any;
     let errorFn = (err: any): any => null;
@@ -155,3 +164,11 @@ export class EventEmitter<T> extends Subject<T> {
     return sink;
   }
 }
+
+/**
+ * @publicApi
+ */
+export const EventEmitter: {
+  new (isAsync?: boolean): EventEmitter<any>; new<T>(isAsync?: boolean): EventEmitter<T>;
+  readonly prototype: EventEmitter<any>;
+} = EventEmitter_ as any;