Initial support for length guards on final fields.

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 bd795ce8987062228c05262ed2f10f1381d2ecb5..ab171982285098ee31dabb8e3fe243a8dd218e71 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -821,6 +821,7 @@ RawField* Field::ReadFrom(SnapshotReader* reader,
   field.set_token_pos(reader->ReadIntptrValue());
   field.set_guarded_cid(reader->ReadIntptrValue());
   field.set_is_nullable(reader->ReadIntptrValue());
+  field.set_guarded_list_length(reader->ReadIntptrValue());
   field.set_kind_bits(reader->Read<uint8_t>());
 
   // Set all the object fields.
@@ -854,6 +855,7 @@ void RawField::WriteTo(SnapshotWriter* writer,
   writer->WriteIntptrValue(ptr()->token_pos_);
   writer->WriteIntptrValue(ptr()->guarded_cid_);
   writer->WriteIntptrValue(ptr()->is_nullable_);
+  writer->WriteIntptrValue(ptr()->guarded_list_length_);
   writer->Write<uint8_t>(ptr()->kind_bits_);
 
   // Write out all the object pointer fields.