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..f8af9df2121874442e75eddf8813e84393e05352 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -6680,7 +6680,7 @@ class String : public Instance {
   // an Array object or a regular Object so that it can be traversed during
   // garbage collection.
   RawString* MakeExternal(void* array,
-                          intptr_t length,
+                          intptr_t external_size,
                           void* peer,
                           Dart_PeerFinalizer cback) const;
 
@@ -6936,6 +6936,7 @@ class OneByteString : public AllStatic {
                                               Heap::Space space);
 
   static void SetPeer(const String& str,
+                      intptr_t external_size,
                       void* peer,
                       Dart_PeerFinalizer cback);
 
@@ -7051,6 +7052,7 @@ class TwoByteString : public AllStatic {
                                      Heap::Space space);
 
   static void SetPeer(const String& str,
+                      intptr_t external_size,
                       void* peer,
                       Dart_PeerFinalizer cback);
 
@@ -7896,7 +7898,9 @@ 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_);