How to add //@dart=2.9 in dart file

Question

Now I'm working in flutter project. I need to add //@dart=2.9 in dart file. But i got run time error. Like A library can't opt out of null safety by default,when using sound null safety. How can i add it on dart file?

Answer 1

add it at the top of main.dart.

then run $ flutter clean $ flutter pub get

if this does not work,check this https://dart.dev/null-safety/migration-guide

Answer 2

Use like this

//@dart=2.9
Import 'package: flutter/material. Dart';

class dart extends StatefulWidget {
  dart ({Key key}): super (key: key);
  @override
  _dartState createState () => _dartState();
}

class _dartState extends State<dart> {
  @override
  Widget builds (BuildContext context) => Scaffold();
}