constructor() {
    /**
     * A pull method that is called when the ReadableStream's internal queue
     * becomes not full.
     * @type {(function(!ReadableStreamDefaultController<T>):
     *     (!Promise<undefined>|undefined))|undefined}
     */
    this.pull;

    /**
     * Called when the ReadableStream is cancelled.
     * @type {(function(*): (!Promise<undefined>|undefined))|undefined}
     */
    this.cancel;
  }
The underlying source for a ReadableStream.
@template T
@record
@extends {liteTypes.ReadableStreamUnderlyingSource}