src/vs/base/common/observableImpl/utils.ts/FromEventObservable/get
class FromEventObservable:	public get(): T {
		if (this.subscription) {
			if (!this.hasValue) {
				this.handleEvent(undefined);
			}
			return this.value!;
		} else {
			// no cache, as there are no subscribers to keep it updated
			return this.getValue(undefined);
		}
	}