Address Florian's comments.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 13424)
+++ runtime/vm/object.cc	(working copy)
@@ -7401,6 +7401,7 @@
 }
 
 
+#if defined(DEBUG)
 // Used in asserts to verify that a check is not added twice.
 bool ICData::HasCheck(const GrowableArray<intptr_t>& cids) const {
   for (intptr_t i = 0; i < NumberOfChecks(); i++) {
@@ -7420,11 +7421,12 @@
   }
   return false;
 }
+#endif  // DEBUG
 
 
 void ICData::AddCheck(const GrowableArray<intptr_t>& class_ids,
                       const Function& target) const {
-  ASSERT(!HasCheck(class_ids));
+  DEBUG_ASSERT(!HasCheck(class_ids));
   ASSERT(num_args_tested() > 1);  // Otherwise use 'AddReceiverCheck'.
   ASSERT(class_ids.length() == num_args_tested());
   const intptr_t old_num = NumberOfChecks();