Change heuristic to determine which fields contain modifieable double boxes: do it if field was ini…

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index ce8233fedd6a93e554ae9f2fd7be917ce1697ec2..88ebd0e8e941309c6b21a740741755ce938ac85a 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -7229,6 +7229,7 @@ RawField* Field::New(const String& name,
   result.set_is_final(is_final);
   result.set_is_const(is_const);
   result.set_is_reflectable(is_reflectable);
+  result.set_is_double_initialized(false);
   result.set_owner(owner);
   result.set_token_pos(token_pos);
   result.set_has_initializer(false);