fix #27258, don't allow dynamic set of a final field

tests/language_strong/dynamic_set_test.dart

+// Copyright (c) 2012, 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 that ensures that fields can be accessed dynamically.
+
+import "package:expect/expect.dart";
+
+class C {
+  final x = "hello";
+  get y => ", ";
+  m() => "world!";
+}
+
+// Regression test for https://github.com/dart-lang/sdk/issues/27258
+main() {
+  dynamic c = new C();
+  Expect.equals(c.x + c.y + c.m(), "hello, world!");
+
+  Expect.throws(() {
+    c.x = 1;
+  });
+  Expect.throws(() {
+    c.x = '1';
+  });
+  Expect.throws(() {
+    c.y = '2';
+  });
+  Expect.throws(() {
+    c.m = '3';
+  });
+
+  Expect.equals(c.x + c.y + c.m(), "hello, world!");
+}