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

tests/language_strong/mock_writable_final_field_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 'package:expect/expect.dart';
+
+final values = <int>[];
+
+class Mock {
+  noSuchMethod(Invocation i) {
+    Expect.equals(i.memberName.toString(), 'Symbol("_x")');
+    values.add(i.positionalArguments[0]);
+  }
+}
+
+class Foo {
+  int _x;
+}
+
+class Bar extends Mock implements Foo {
+  final int _x = 42;
+}
+
+void main() {
+  {
+    Bar b = new Bar();
+    Expect.equals(b._x, 42);
+    b._x = 123;
+    Expect.listEquals(values, [123]);
+    values.clear();
+  }
+  {
+    // It works the same if called statically through the Foo interface.
+    Foo b = new Bar();
+    Expect.equals(b._x, 42);
+    b._x = 123;
+    Expect.listEquals(values, [123]);
+    values.clear();
+  }
+}