Lint rule: Do not override fields.

test/rules/final_override_non_final.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// test w/ `dart test/util/solo_test.dart final_override_non_final`
+
+class Base {
+  Object field = 'lorem';
+
+  Object something = 'change';
+}
+
+class Bad1 extends Base {
+  @override
+  final field = 'ipsum'; // LINT
+}
+
+class Bad2 extends Base {
+  @override
+  Object something = 'done'; // LINT
+}
+
+class Ok extends Base {
+  Object newField; // OK
+
+  final Object newFinal = 'ignore'; // OK
+}