src/vs/workbench/api/common/extHostMemento.ts/ExtensionMemento/update
class ExtensionMemento:	update(key: string, value: any): Promise<void> {
		this._value![key] = value;

		const record = this._deferredPromises.get(key);
		if (record !== undefined) {
			return record.p;
		}

		const promise = new DeferredPromise<void>();
		this._deferredPromises.set(key, promise);

		if (!this._scheduler.isScheduled()) {
			this._scheduler.schedule();
		}

		return promise.p;
	}