constructor(reader, preventCancel) {
		            this._ongoingPromise = undefined;
		            this._isFinished = false;
		            this._reader = reader;
		            this._preventCancel = preventCancel;
		        }
Cancels the stream, signaling a loss of interest in the stream by a consumer.

The supplied `reason` argument will be given to the underlying source's {@link UnderlyingSource.cancel | cancel()}
method, which might or might not use it.