Reapplies (again) "Don't try to box immutable objects."

tests/language/src/Closure6Test.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 captured final variables are correctly mangled.
+
+class A {
+  foo() {
+    length() => 400;
+    final box_0 = 28;
+    var x = 29;
+    var f = () => length() + box_0 + x + bar();
+    return f();
+  }
+  bar() => 42;
+}
+
+main() {
+  Expect.equals(499, new A().foo());
+}