FlatButton, RaisedButton, and OutlineButton have been replaced by TextButton,
ElevatedButton, and OutlinedButton respectively. 

ButtonTheme has been replaced by TextButtonTheme, ElevatedButtonTheme, and
OutlinedButtonTheme. The original classes will be deprecated soon, please 
migrate code that uses them. There's a detailed migration guide for the new 
button and button theme classes in flutter.

dev/go/material-button-migration-guide.

refrence: 
https://api.flutter.dev/flutter/material/RaisedButton-class.html