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

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 76d49edd8a1dd11c3083b92b672c4c4bada72c92..c30deb7cb6288b0e3b812922d6514c59cee5d593 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -881,7 +881,7 @@ class RawField : public RawObject {
   RawObject** from() { return reinterpret_cast<RawObject**>(&ptr()->name_); }
   RawString* name_;
   RawObject* owner_;  // Class or patch class or mixin class
-                      // where this field is defined.
+                      // where this field is defined or original field.
   RawAbstractType* type_;
   union {
     RawInstance* static_value_;  // Value for static fields.