Put functions used in compile-time constants in the worklist of codegen.

tests/language/methods_as_constants2_test.dart

Index: tests/language/methods_as_constants2_test.dart
===================================================================
--- tests/language/methods_as_constants2_test.dart	(revision 0)
+++ tests/language/methods_as_constants2_test.dart	(revision 0)
@@ -0,0 +1,15 @@
+// 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 a function only used by compile-time constants is being
+// generated.
+
+topLevelMethod() => 42;
+
+class A {
+  final Function f;
+  const A(this.f);
+}
+main() {
+  Expect.equals(42, const A(topLevelMethod).f());
+}