src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeViews/convertArgument
class ExtHostTreeViews:	private convertArgument(arg: TreeViewItemHandleArg): any {
		const treeView = this.treeViews.get(arg.$treeViewId);
		return treeView ? treeView.getExtensionElement(arg.$treeItemHandle) : null;
	}