Assign external sizes to external strings and to external typed data created by the VM.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index b219ab7c5cdc3cbaee33f1d3eb32ac751c4469fc..06ae3c672c4ef1d903f03c4ce319362ccc5829a6 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -7896,7 +7896,9 @@ class ExternalTypedData : public Instance {
 #undef TYPED_GETTER_SETTER
 
   FinalizablePersistentHandle* AddFinalizer(
-      void* peer, Dart_WeakPersistentHandleFinalizer callback) const;
+      void* peer,
+      Dart_WeakPersistentHandleFinalizer callback,
+      intptr_t external_size) const;
 
   static intptr_t length_offset() {
     return OFFSET_OF(RawExternalTypedData, length_);