Errors like these
/C:/src/flutter/flutter/.pub-cache/hosted/pub.dartlang.org/firebase_database-6.1.0/lib/src/firebase_database.dart:14:26: Error: The parameter 'app' can't have a value of 'null' because of its type 'FirebaseApp', but the implicit default value is 'null'.
- 'FirebaseApp' is from 'package:firebase_core/firebase_core.dart' ('/C:/src/flutter/flutter/.pub-cache/hosted/pub.dartlang.org/firebase_core-1.0.1/lib/firebase_core.dart'). Try adding either an explicit non-'null' default value or the 'required' modifier.