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() {
+  void fun(x) {
+    if (x < 3) {
+      return foo((x) => fun(x));
+    } else {
+      return x;
+    }
+  }
+  Expect.equals(499, foo((x) => fun(x)));
+}