In background compilation make a copy of Field in order to freeze its state

runtime/vm/flag_list.h

Index: runtime/vm/flag_list.h
diff --git a/runtime/vm/flag_list.h b/runtime/vm/flag_list.h
index 570d0fc37e4db07e993659571a7a64e114a06aeb..a62d1ec62472aa842f75033421abfdde6f47634f 100644
--- a/runtime/vm/flag_list.h
+++ b/runtime/vm/flag_list.h
@@ -51,6 +51,8 @@ C(enable_mirrors, false, false, bool, true,                                    \
   "Disable to make importing dart:mirrors an error.")                          \
 C(fields_may_be_reset, true, false, bool, false,                               \
   "Don't optimize away static field initialization")                           \
+C(force_clone_compiler_objects, false, false, bool, false,                     \
+  "Force cloning of objects needed in compiler (ICData and Field).")           \
 R(gc_at_alloc, false, bool, false,                                             \
   "GC at every allocation.")                                                   \
 P(getter_setter_ratio, int, 13,                                                \