message: refactor(mapTo): smaller implementation
diff: import { OperatorFunction } from '../types';
  -import { operate } from '../util/lift';
  -import { OperatorSubscriber } from './OperatorSubscriber';
  +import { map } from './map';
  export function mapTo<R>(value: R): OperatorFunction<any, R>;
  /** @deprecated Do not specify explicit type parameters. Signatures with type parameters that cannot be inferred will be removed in v8. */
  @@ -32,19 +31,10 @@ export function mapTo<T, R>(value: R): OperatorFunction<T, R>;
  *
  * @see {@link map}
  *
  - * @param {any} value The value to map each source value to.
  + * @param value The value to map each source value to.
  * @return A function that returns an Observable that emits the given `value`
  * every time the source Observable emits.
  */
  export function mapTo<R>(value: R): OperatorFunction<any, R> {
  - return operate((source, subscriber) => {
  - // Subscribe to the source. All errors and completions are forwarded to the consumer
  - source.subscribe(
  - new OperatorSubscriber(
  - subscriber,
  - // On every value from the source, send the `mapTo` value to the consumer.
  - () => subscriber.next(value)
  - )
  - );
  - });
  + return map(() => value);
  }
  