OAuth2 authentication in Flutter

I can not authenticate with Flutter and OAuth2. Basically I created a Flutter base project and added the example of the OAuth2 wiki.

This is the class from Flutter basic project:

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _login() async {
    final authorizationEndpoint =
    Uri.parse("http://blablablabla/o/token");

    final username = "username";
    final password = "password";

    // Something like this
    final identifier = "mavEZjJgs9d4JwvvXsANZgN5Dz5GFxzfj616752A";
    final secret = "hcAFRwvtGqzhKrMPH2Vqm1vncuZt2YTVfTs6LcdNcnKPdEH3J0T1njIwurryofrvDMnzOvhQDVbaC9Gt5DctciTv3n89s7JSGjpHtzkbEfLpkOT5y6YHN3p6grQlYGd59";

    // Make a request to the authorization endpoint that will produce the fully
    // authenticated Client.
    var client = await oauth2.resourceOwnerPasswordGrant(
        authorizationEndpoint, username, password,
        identifier: identifier, secret: secret);

    // Once you have the client, you can use it just like any other HTTP client.
    var result = await client.read("http://blablabla/api/users/me/");

    // Once we're done with the client, save the credentials file. This will allow
    // us to re-use the credentials and avoid storing the username and password
    // directly.
    new File("~/.myapp/credentials.json")
        .writeAsString(client.credentials.toJson());
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text(
              'You have pushed the button this many times:',
            ),
            new Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
//        onPressed: _incrementCounter,
        onPressed: _login,
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

I changed the _incrementCounter method of the tutorial to _login. When I press the button (which I should now log me in) I get the following:

E/flutter ( 7662): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter ( 7662): NoSuchMethodError: No top-level method 'base64Encode' declared.
E/flutter ( 7662): Receiver: top-level
E/flutter ( 7662): Tried calling: base64Encode(Uint8Array)
E/flutter ( 7662): #0      NoSuchMethodError._throwNew (dart:core-patch/dart:core/errors_patch.dart:192)
E/flutter ( 7662): #1      basicAuthHeader (package:oauth2/src/utils.dart:14)
E/flutter ( 7662): #2      resourceOwnerPasswordGrant (package:oauth2/src/resource_owner_password_grant.dart:69)
E/flutter ( 7662): <asynchronous suspension>
E/flutter ( 7662): #3      _MyHomePageState._login (file:///home/thisismyuser/AndroidStudioProjects/login/lib/main.dart:79)
E/flutter ( 7662): <asynchronous suspension>
E/flutter ( 7662): #4      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:478)
E/flutter ( 7662): #5      _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:530)
E/flutter ( 7662): #6      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:102)
E/flutter ( 7662): #7      TapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:161)
E/flutter ( 7662): #8      TapGestureRecognizer.acceptGesture (package:flutter/src/gestures/tap.dart:123)
E/flutter ( 7662): #9      GestureArenaManager.sweep (package:flutter/src/gestures/arena.dart:156)
E/flutter ( 7662): #10     BindingBase&GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:147)
E/flutter ( 7662): #11     BindingBase&GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:121)
E/flutter ( 7662): #12     BindingBase&GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:101)
E/flutter ( 7662): #13     BindingBase&GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:64)
E/flutter ( 7662): #14     BindingBase&GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:48)
E/flutter ( 7662): #15     _invoke1 (file:///b/build/slave/Linux_Engine/build/src/flutter/lib/ui/hooks.dart:134)
E/flutter ( 7662): #16     _dispatchPointerDataPacket (file:///b/build/slave/Linux_Engine/build/src/flutter/lib/ui/hooks.dart:91)

Thanks in advance!

(With Postman I get this beautiful authentication)