Externalize test code

Change-Id: Iab19397c7a72fb9a3ca63bfd0bc4eaf1a98138ba
diff --git a/src/image_test.cc b/src/image_test.cc
index 54c2c75..bd30e71 100644
--- a/src/image_test.cc
+++ b/src/image_test.cc
@@ -51,7 +51,7 @@
   // lucky by pointers that happen to work referencing the earlier
   // dex.
   delete java_lang_dex_file_.release();
-  scoped_ptr<DexFile> dex(GetLibCoreDex());
+  scoped_ptr<const DexFile> dex(GetLibCoreDex());
   ASSERT_TRUE(dex != NULL);
 
   std::vector<const DexFile*> boot_class_path;