Revert "Reapply "Don't try to box immutable objects."

tests/language/src/Closure5Test.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.
-
-// Dart2js failed when a declared function was captured inside itself.
-
-foo(f) => f(499);
-
-main() {
-  fun(x) {
-    if (x < 3) {
-      return foo((x) => fun(x));
-    } else {
-      return x;
-    }
-  }
-  Expect.equals(499, foo((x) => fun(x)));
-}