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

runtime/vm/jit_optimizer.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/jit_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/branch_optimizer.h"
 #include "vm/cha.h"
 #include "vm/compiler.h"
@@ skipping 2970 matching lines @@
 
 void JitOptimizer::VisitStoreInstanceField(
     StoreInstanceFieldInstr* instr) {
   if (instr->IsUnboxedStore()) {
     ASSERT(instr->is_potential_unboxed_initialization_);
     // Determine if this field should be unboxed based on the usage of getter
     // and setter functions: The heuristic requires that the setter has a
     // usage count of at least 1/kGetterSetterRatio of the getter usage count.
     // This is to avoid unboxing fields where the setter is never or rarely
     // executed.
-    const Field& field = Field::ZoneHandle(Z, instr->field().raw());
+    const Field& field = Field::ZoneHandle(Z, instr->field().Original());
     const String& field_name = String::Handle(Z, field.name());
-    const Class& owner = Class::Handle(Z, field.owner());
+    const Class& owner = Class::Handle(Z, field.Owner());
     const Function& getter =
         Function::Handle(Z, owner.LookupGetterFunction(field_name));
     const Function& setter =
         Function::Handle(Z, owner.LookupSetterFunction(field_name));
     bool unboxed_field = false;
     if (!getter.IsNull() && !setter.IsNull()) {
       if (field.is_double_initialized()) {
         unboxed_field = true;
       } else if ((setter.usage_counter() > 0) &&
                  ((FLAG_getter_setter_ratio * setter.usage_counter()) >=
@@ skipping 144 matching lines @@
 
   // Discard the environment from the original instruction because the store
   // can't deoptimize.
   instr->RemoveEnvironment();
   ReplaceCall(instr, store);
   return true;
 }
 
 
 }  // namespace dart