src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeView/refresh
class ExtHostTreeView:	private refresh(elements: (T | Root)[]): Promise<void> {
		const hasRoot = elements.some(element => !element);
		if (hasRoot) {
			// Cancel any pending children fetches
			this._refreshCancellationSource.dispose(true);
			this._refreshCancellationSource = new CancellationTokenSource();

			this.clearAll(); // clear cache
			return this.proxy.$refresh(this.viewId);
		} else {
			const handlesToRefresh = this.getHandlesToRefresh(<T[]>elements);
			if (handlesToRefresh.length) {
				return this.refreshHandles(handlesToRefresh);
			}
		}
		return Promise.resolve(undefined);
	}