src/vs/workbench/contrib/files/browser/views/openEditorsView.ts/OpenEditorsDragAndDrop/getDragURI
class OpenEditorsDragAndDrop:	getDragURI(element: OpenEditor | IEditorGroup): string | null {
		if (element instanceof OpenEditor) {
			const resource = element.getResource();
			if (resource) {
				return resource.toString();
			}
		}
		return null;
	}