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 83e45e9cb2ef0728c0bf1d08c1b85559c9724d77..2d50bc099573b8404a759f2b7000a7355c3d06a2 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_synthetic(is_synthetic);
+  result.set_is_double_initialized(false);
   result.set_owner(owner);
   result.set_token_pos(token_pos);
   result.set_has_initializer(false);