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

runtime/vm/raw_object_snapshot.cc

Index: runtime/vm/raw_object_snapshot.cc
diff --git a/runtime/vm/raw_object_snapshot.cc b/runtime/vm/raw_object_snapshot.cc
index d94ce4e2e4e509fe31340bced5b79bcc7e065f41..361287631b24374fa300c2e0c42fcfecd0215602 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -2634,7 +2634,8 @@ RawExternalTypedData* ExternalTypedData::ReadFrom(SnapshotReader* reader,
   Dart_WeakPersistentHandleFinalizer callback =
       reinterpret_cast<Dart_WeakPersistentHandleFinalizer>(
           reader->ReadRawPointerValue());
-  obj.AddFinalizer(peer, callback);
+  intptr_t external_size = obj.LengthInBytes();
+  obj.AddFinalizer(peer, callback, external_size);
   return obj.raw();

[siva, 2016/08/29 23:03:45]

This whole function should be a NOP or UNREACHABLE right because we never
serialize an ExternalTypedData object as an ExternalTypedData object right?

[rmacnak, 2016/08/30 20:49:17]

This is never generated from raw_snapshot_object.cc, but it is generated from
PostCObject.

 }