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 6c634c8974ad3a0077052ae7e86032bafcc1e885..c30823f1ec5cf648e17a2e3ee2a25ee730840404 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -424,6 +424,9 @@ class RawObject {
   bool IsScript() {
     return ((GetClassId() == kScriptCid));
   }
+  bool IsField() {
+    return ((GetClassId() == kFieldCid));
+  }
   bool IsFunction() {
     return ((GetClassId() == kFunctionCid));
   }
@@ -881,7 +884,7 @@ 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.