"flutter_platform_widgets" Dependency Error

[AyberkAhmet]

/C:/Users/Ayberk.A/AppData/Local/Pub/Cache/hosted/pub.dev/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' ('/C:/Users/Ayberk.A/AppData/Local/Pub/Cache/hosted/pub.dev/flutter_platform_widgets-1.20.0/lib/src/platform_button.dart').
platform_button.dart:1
Try correcting the name to the name of an existing method, or defining a method named 'FlatButton'.
return FlatButton(

/C:/Users/Ayberk.A/AppData/Local/Pub/Cache/hosted/pub.dev/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' ('/C:/Users/Ayberk.A/AppData/Local/Pub/Cache/hosted/pub.dev/flutter_platform_widgets-1.20.0/lib/src/platform_button.dart').
platform_button.dart:1
Try correcting the name to the name of an existing method, or defining a method named 'RaisedButton'.
return RaisedButton(

/C:/Users/Ayberk.A/AppData/Local/Pub/Cache/hosted/pub.dev/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' ('/C:/Users/Ayberk.A/AppData/Local/Pub/Cache/hosted/pub.dev/flutter_platform_widgets-1.20.0/lib/src/platform_dialog_action.dart').
platform_dialog_action.dart:1
Try correcting the name to the name of an existing method, or defining a method named 'FlatButton'.
return FlatButton(