fix #29233, final fields can be settable in a mock

tests/language_strong/mixin_and_extension_member_test.dart

+// Copyright (c) 2017, 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.
+
+import 'dart:collection';
+import 'package:expect/expect.dart';
+
+class OverrideFirstGetter {
+  get first => 9999;
+}
+
+class ListMock extends ListBase with OverrideFirstGetter {
+  final _list = [];
+  int get length => _list.length;
+  void set length(int x) {
+    _list.length = x;
+  }
+
+  operator [](x) => _list[x];
+  void operator []=(x, y) {
+    _list[x] = y;
+  }
+}
+
+// Regression test for
+// https://github.com/dart-lang/sdk/issues/29273#issuecomment-292384130
+main() {
+  List x = new ListMock();
+  x.add(42);
+  Expect.equals(x[0], 42);
+  Expect.equals(x.first, 9999);
+}