message: rmuf: Don't delete writes to external fields
  Summary:
  Apparently can_delete / can_rename will return true for external fields...
  seems like we should fix that API sometime...
diff: @@ -44,7 +44,7 @@ void PassImpl::run_pass(DexStoresVector& stores,
  return;
  }
  // XXX(jezng): why is can_rename not a subset of can_delete?
  - if (!can_delete(field) || !can_rename(field)) {
  + if (field->is_external() || !can_delete(field) || !can_rename(field)) {
  return;
  }
  if (is_sget(op) || is_iget(op)) {
  