我得到这些错误，不知道如何修复它们。

../AppData/Local/Pub/Cache/hosted/pub.dartlang.org/flutter_platform_widgets-1.20.0/lib/src/platform_button.dart:269:14: Error: The method 'FlatButton' isn't defined for the class 'PlatformButton'.
 - 'PlatformButton' is from 'package:flutter_platform_widgets/src/platform_button.dart' ('../AppData/Local/Pub/Cache/hosted/pub.dartlang.org/flutter_platform_widgets-1.20.0/lib/src/platform_button.dart').
Try correcting the name to the name of an existing method, or defining a method named 'FlatButton'.
      return FlatButton(
             ^^^^^^^^^^

../AppData/Local/Pub/Cache/hosted/pub.dartlang.org/flutter_platform_widgets-1.20.0/lib/src/platform_button.dart:302:12: Error: The method 'RaisedButton' isn't defined for the class 'PlatformButton'.
 - 'PlatformButton' is from 'package:flutter_platform_widgets/src/platform_button.dart' ('../AppData/Local/Pub/Cache/hosted/pub.dartlang.org/flutter_platform_widgets-1.20.0/lib/src/platform_button.dart').
Try correcting the name to the name of an existing method, or defining a method named 'RaisedButton'.
    return RaisedButton(
           ^^^^^^^^^^^^

../AppData/Local/Pub/Cache/hosted/pub.dartlang.org/flutter_platform_widgets-1.20.0/lib/src/platform_dialog_action.dart:171:14: Error: The method 'FlatButton' isn't defined for the class 'PlatformDialogAction'.
 - 'PlatformDialogAction' is from 'package:flutter_platform_widgets/src/platform_dialog_action.dart' ('../AppData/Local/Pub/Cache/hosted/pub.dartlang.org/flutter_platform_widgets-1.20.0/lib/src/platform_dialog_action.dart').
Try correcting the name to the name of an existing method, or defining a method named 'FlatButton'.
      return FlatButton(
             ^^^^^^^^^^