src/vs/platform/windows/electron-main/windowsMainService.ts/WindowsMainService/doAddFoldersToExistingWindow
class WindowsMainService:	private doAddFoldersToExistingWindow(window: ICodeWindow, foldersToAdd: URI[]): ICodeWindow {
		this.logService.trace('windowsManager#doAddFoldersToExistingWindow');

		window.focus(); // make sure window has focus

		const request: IAddFoldersRequest = { foldersToAdd };
		window.sendWhenReady('vscode:addFolders', CancellationToken.None, request);

		return window;
	}