我正在使用DartVMversion:1.24.3(WedDec1323:26:592017)on"macos_x64"和Flutter0.2.8•channelbetaFramework•revisionb397406561(13daysago)Engine•revisionc903c217a1Tools•Dart2.0.0-dev.43.0.flutter-52afcba357ReadAsStringSync给我一个空字符串作为输出而不是所需的文件内容，我的代码是varappl=newFile('a.txt').readAsStringSync();print(appl);我还需要