async data source can be sync

src/vs/base/browser/ui/tree/asyncDataTree.ts

@@ -468,7 +468,7 @@ export class AsyncDataTree<T extends NonNullable<any>, TFilterData = void> imple
 				this._onDidChangeNodeState.fire(node);
 			}, _ => null);
 
-			return this.dataSource.getChildren(node.element)
+			return Promise.resolve(this.dataSource.getChildren(node.element))
 				.then(children => {
 					slowTimeout.cancel();
 					node.state = AsyncDataTreeNodeState.Loaded;

src/vs/base/browser/ui/tree/tree.ts

@@ -147,7 +147,7 @@ export interface ITreeNavigator<T> {
 
 export interface IAsyncDataSource<T extends NonNullable<any>> {
 	hasChildren(element: T | null): boolean;
-	getChildren(element: T | null): Promise<T[]>;
+	getChildren(element: T | null): T[] | Promise<T[]>;
 }
 
 /**