Exemplo do github não fuincionou.

Tive que impementar da seguinte maneira:

class FormularioTransferencia extends StatelessWidget {
  final TextEditingController _controladorCampoNumeroConta =
      TextEditingController();
  final TextEditingController _controladorCampoValor = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Formulario Transferencia"),
      ),
//CHAMANDO UM BUILDER EM VEZ DO COLUMN
      body: Builder(
        builder: (BuildContext ctx){ //CONTEXTO DO BUILDER
          return Column(
            children: <Widget>[
              Padding(
                padding: const EdgeInsets.all(16.0),
                child: TextField(
                  controller: _controladorCampoNumeroConta,
                  style: TextStyle(fontSize: 24.0),
                  decoration: InputDecoration(
                      labelText: "Número da conta", hintText: "0000"),
                  keyboardType: TextInputType.number,
                ),
              ),
              Padding(
                padding: const EdgeInsets.all(16.0),
                child: TextField(
                  controller: _controladorCampoValor,
                  style: TextStyle(fontSize: 24.0),
                  decoration: InputDecoration(
                      icon: Icon(Icons.monetization_on),
                      labelText: "Valor",
                      hintText: "0.00"),
                  keyboardType: TextInputType.number,
                ),
              ),
              RaisedButton(
                child: Text("Confirmar"),
                onPressed: () {
                  final int conta = int.tryParse(_controladorCampoNumeroConta.text);
                  final double valor = double.tryParse(_controladorCampoValor.text);

                  if (valor != null && conta != null) {
                    final transferenciaCriada = Transferencia(valor, conta);
                    debugPrint("$transferenciaCriada");

            //CONTEXTO DO BUILDER
                    Scaffold.of(ctx).showSnackBar(SnackBar(
                      content: Text("$transferenciaCriada"),
                    ));

                  }
                },
              )
            ],
          );
        },
      ),
    );
  }
}