src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeViews/$handleDrag
class ExtHostTreeViews:	async $handleDrag(sourceViewId: string, sourceTreeItemHandles: string[], operationUuid: string, token: CancellationToken): Promise<DataTransferDTO | undefined> {
		const treeView = this.treeViews.get(sourceViewId);
		if (!treeView) {
			return Promise.reject(new Error(localize('treeView.notRegistered', 'No tree view with id \'{0}\' registered.', sourceViewId)));
		}

		const treeDataTransfer = await this.addAdditionalTransferItems(new types.DataTransfer(), treeView, sourceTreeItemHandles, token, operationUuid);
		if (!treeDataTransfer) {
			return;
		}

		return DataTransfer.toDataTransferDTO(treeDataTransfer);
	}