Issue getting flutter app to load

main.dart

// bloc.dart
 import 'dart:async';
 import 'validators.dart';

class Bloc extends Validators {
  final _email = new StreamController<String>();
  final _password = new StreamController<String>();

  // Add data to stream
  Stream<String> get email => _email.stream.transform(validateEmail);
  Stream<String> get password => _password.stream.transform(validatePassword);

  // Change Data
  Function(String) get changeEmail => _email.sink.add;
  Function(String) get changePassword => _password.sink.add;

  dispose() {
    _email.close();
    _password.close();
  }
}

final bloc = Bloc();

// login_screen.dart
import 'package:flutter/material.dart';
import '../blocs/bloc.dart';

class LoginScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
        margin: EdgeInsets.all(20.0),
        child: Column(
          children: [
            emailField(),
            passwordField(),
            Container(
              margin: EdgeInsets.only(top: 25.0),
            ),
            raisedButton()
          ],
        ));
  }

  Widget emailField() {
    return StreamBuilder(
        stream: bloc.email,
        builder: (context, snapshot) {
          return TextField(
            onChanged: bloc.changeEmail,
            keyboardType: TextInputType.emailAddress,
            decoration: InputDecoration(
                hintText: 'ardon@exampl.com',
                labelText: 'Enter your email!',
                errorText: snapshot.error),
          );
        });
  }

  Widget passwordField() {
    return TextField(
      obscureText: true,
      decoration: InputDecoration(hintText: 'password', labelText: 'password'),
    );
  }

  Widget raisedButton() {
    return RaisedButton(
      child: Text('Login'),
      color: Colors.blue,
      onPressed: () {},
    );
  }
}

// validation.dart
import 'dart:async';

class Validators {
  final validateEmail =
      StreamTransformer<String, String>.fromHandlers(handleData: (email, sink) {
    if (email.contains('@')) {
      sink.add(email);
    } else {
      sink.addError('Please enter valid email');
    }
  });

  final validatePassword = StreamTransformer<String, String>.fromHandlers(
      handleData: (password, sink) {
    if (password.length > 4) {
      sink.add(password);
    } else {
      sink.addError('Please enter the right number of characters');
    }
  });
}