fix(Subscriber): next method no longer has optional value argument

resolves ReactiveX#2852

spec-dtslint/Observable-spec.ts

@@ -131,4 +131,11 @@ describe('pipe', () => {
     const o2 = of(123).pipe(map(n => n + '?'), source => source.subscribe()); // $ExpectType Subscription
     const o3 = of('test').pipe(map(n => n + ':' + n), filter(n => n < 30)); // $ExpectError
   })
+});
+
+it('should provide the proper types to the subscriber', () => {
+  const o1$ = new Observable<number>(subscriber => {
+    const next = subscriber.next; // $ExpectType (value: number) => void
+    subscriber.next(); // $ExpectError
+  });
 });

src/internal/Subscriber.ts

@@ -123,7 +123,7 @@ export class Subscriber<T> extends Subscription implements Observer<T> {
    * times.
    * @param value The `next` value.
    */
-  next(value?: T): void {
+  next(value: T): void {
     if (this.isStopped) {
       handleStoppedNotification(nextNotification(value), this);
     } else {