waitFor(item, callback) {
		const key = this._getKey(item);
		const entry = this._entries.get(key);
		if (entry === undefined) {
			return callback(
				new WebpackError(
					"waitFor can only be called for an already started item"
				)
			);
		}
		if (entry.state === DONE_STATE) {
			process.nextTick(() => callback(entry.error, entry.result));
		} else if (entry.callbacks === undefined) {
			entry.callbacks = [callback];
		} else {
			entry.callbacks.push(callback);
		}
	}
Waits for an already started item
@param {T} item an item
@param {Callback<R>} callback callback function
@returns {void}