src/vs/workbench/common/notifications.ts/NotificationViewItem/progress
class NotificationViewItem:	get progress(): INotificationViewItemProgress {
		if (!this._progress) {
			this._progress = this._register(new NotificationViewItemProgress());
			this._register(this._progress.onDidChange(() => this._onDidChangeContent.fire({ kind: NotificationViewItemContentChangeKind.PROGRESS })));
		}

		return this._progress;
	}