class FirstPage extends StatelessWidget{
final String firstText;
FirstPage(this.firstText);

Widget build(BuildContext context){
return RaisedButton(onPressed: (){
    firstText = "Hello World";
    Navigator.of(context).push(SecondPage(secondText: firstText); //The firstText value is now passed to SecondPage class
    }
}

class SecondPage extends StatelessWidget{
final String secondText;//instantiate
SecondPage(this.secondText);//constructor

Widget build(BuildContext context){
return Text(seconText);//will have the value of "firstText" which is "hello world"