Add new flutter example of todo app with provider (#78)

todo_list_using_provider/lib/notifiers/todo_list.dart

@@ -0,0 +1,25 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter_new_provider_todo/models/todo.dart';
+
+/// This class represents a model
+/// that notify listeners when it has some changes
+class TodoList extends ChangeNotifier {
+  List<Todo> _list = new List<Todo>();
+
+  List<Todo> get list => _list;
+
+  void add(Todo newItem) {
+    this._list.add(newItem);
+    notifyListeners();
+  }
+
+  void remove(Todo removedItem) {
+    this._list.remove(removedItem);
+    notifyListeners();
+  }
+
+  void clearDoneItems() {
+    this._list.removeWhere((Todo element) => element.done);
+    notifyListeners();
+  }
+}