constructor() {
    /**
     * A start method that is called when the ReadableStream is constructed.
     *
     * For the purpose of the lite version, this method is not optional,
     * and the return value is not used. In other versions, a Promise return
     * value will prevent calls to pull until the Promise is resolved.
     * @type {(function(!ReadableStreamDefaultController<T>):
     *     (!Promise<undefined>|undefined))|undefined}
     * https://streams.spec.whatwg.org/#dom-underlying-source-start
     */
    this.start;
  }
The underlying source for a lite ReadableStream.
@template T
@record