Lint rule: Do not override fields.

test/rules/final_override_non_final.dart

Index: test/rules/final_override_non_final.dart
diff --git a/test/rules/final_override_non_final.dart b/test/rules/final_override_non_final.dart
new file mode 100644
index 0000000000000000000000000000000000000000..e9acf9795c81cb55da404e6ddaa97537ed6e0d9d
--- /dev/null
+++ b/test/rules/final_override_non_final.dart
@@ -0,0 +1,27 @@
+// 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
+}