Check that external references are registered in the serializer.

src/serialize.h

Index: src/serialize.h
diff --git a/src/serialize.h b/src/serialize.h
index 8409c951d8c8db9adb6990ee818719ef4f6c263e..5528db30e3711a471e064c451995077bb0a2c73a 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -81,6 +81,11 @@ class ExternalReferenceTable {
   // For other types of references, the caller will figure out the address.
   void Add(Address address, TypeCode type, uint16_t id, const char* name);
 
+  static int Compare(const ExternalReferenceEntry* a,
+                     const ExternalReferenceEntry* b) {
+    return static_cast<int>(a->code) - static_cast<int>(b->code);
+  }
+
   List<ExternalReferenceEntry> refs_;
   int max_id_[kTypeCodeCount];
 };