src/vs/base/common/worker/simpleWorker.ts/SimpleWorkerProtocol/_handleSubscribeEventMessage
class SimpleWorkerProtocol:	private _handleSubscribeEventMessage(msg: SubscribeEventMessage): void {
		const req = msg.req;
		const disposable = this._handler.handleEvent(msg.eventName, msg.arg)((event) => {
			this._send(new EventMessage(this._workerId, req, event));
		});
		this._pendingEvents.set(req, disposable);
	}