src/vs/workbench/api/browser/mainThreadTask.ts/MainThreadTask/$customExecutionComplete
class MainThreadTask:	public $customExecutionComplete(id: string, result?: number): Promise<void> {
		return new Promise<void>((resolve, reject) => {
			this._taskService.getActiveTasks().then((tasks) => {
				for (const task of tasks) {
					if (id === task._id) {
						this._taskService.extensionCallbackTaskComplete(task, result).then((value) => {
							resolve(undefined);
						}, (error) => {
							reject(error);
						});
						return;
					}
				}
				reject(new Error('Task to mark as complete not found'));
			});
		});
	}