How do I open an external url in flutter web in new tab or in same tab

I think you want this — dart:js enables interoperability between Dart and JS —:

import 'dart:js' as js;

// ...

FlatButton(
  child: Text('Button'),
  onPressed: () {
    js.context.callMethod('open', ['https://stackoverflow.com/questions/ask']);
  },
)