message: refactor(mapTo): smaller impl
diff: -import { Operator } from '../Operator';
  +/** @prettier */
  import { Subscriber } from '../Subscriber';
  import { Observable } from '../Observable';
  import { OperatorFunction } from '../types';
  import { lift } from '../util/lift';
  +import { OperatorSubscriber } from './OperatorSubscriber';
  export function mapTo<R>(value: R): OperatorFunction<any, R>;
  /** @deprecated remove in v8. Use mapTo<R>(value: R): OperatorFunction<any, R> signature instead **/
  @@ -40,37 +41,9 @@ export function mapTo<T, R>(value: R): OperatorFunction<T, R>;
  * @name mapTo
  */
  export function mapTo<R>(value: R): OperatorFunction<any, R> {
  - return (source: Observable<any>) => lift(source, new MapToOperator(value));
  -}
  -
  -class MapToOperator<T, R> implements Operator<T, R> {
  -
  - value: R;
  -
  - constructor(value: R) {
  - this.value = value;
  - }
  -
  - call(subscriber: Subscriber<R>, source: any): any {
  - return source.subscribe(new MapToSubscriber(subscriber, this.value));
  - }
  -}
  -
  -/**
  - * We need this JSDoc comment for affecting ESDoc.
  - * @ignore
  - * @extends {Ignored}
  - */
  -class MapToSubscriber<T, R> extends Subscriber<T> {
  -
  - value: R;
  -
  - constructor(destination: Subscriber<R>, value: R) {
  - super(destination);
  - this.value = value;
  - }
  -
  - protected _next(x: T) {
  - this.destination.next(this.value);
  - }
  + return (source: Observable<any>) =>
  + lift(source, function (this: Subscriber<R>, source: Observable<any>) {
  + const subscriber = this;
  + source.subscribe(new OperatorSubscriber(subscriber, () => subscriber.next(value)));
  + });
  }
  