extensions/typescript-language-features/src/tsServer/callbackMap.ts/CallbackMap/add
class CallbackMap:	public add(seq: number, callback: CallbackItem<ServerResponse.Response<R> | undefined>, isAsync: boolean) {
		if (isAsync) {
			this._asyncCallbacks.set(seq, callback);
		} else {
			this._callbacks.set(seq, callback);
		}
	}