Add flag to disable string externalization

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 4f9f8d06a259a357c61a453189db396660bf058f..de97d0bd1759dd6aaea0a02b2c3ec08fdc583f1a 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -65,6 +65,7 @@ DECLARE_FLAG(bool, show_invisible_frames);
 DECLARE_FLAG(bool, trace_deoptimization);
 DECLARE_FLAG(bool, trace_deoptimization_verbose);
 DECLARE_FLAG(bool, write_protect_code);
+DECLARE_FLAG(bool, support_externalizable_strings);
 
 
 static const char* const kGetterPrefix = "get:";
@@ -7815,6 +7816,15 @@ const char* Field::GuardedPropertiesAsCString() const {
 }
 
 
+bool Field::IsExternalizableCid(intptr_t cid) {
+  if (FLAG_support_externalizable_strings) {
+    return (cid == kOneByteStringCid) || (cid == kTwoByteStringCid);
+  } else {
+    return false;
+  }
+}
+
+
 void Field::InitializeGuardedListLengthInObjectOffset() const {
   ASSERT(IsOriginal());
   if (needs_length_check() &&
@@ -19601,6 +19611,7 @@ RawString* String::MakeExternal(void* array,
                                 intptr_t length,
                                 void* peer,
                                 Dart_PeerFinalizer cback) const {
+  ASSERT(FLAG_support_externalizable_strings);
   String& result = String::Handle();
   void* external_data;
   Dart_WeakPersistentHandleFinalizer finalizer;