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

runtime/vm/raw_object.h

 // Copyright (c) 2012, 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.
 
 #ifndef VM_RAW_OBJECT_H_
 #define VM_RAW_OBJECT_H_
 
 #include "platform/assert.h"
 #include "vm/atomic.h"
 #include "vm/globals.h"
@@ skipping 863 matching lines @@
   }
 };
 
 
 class RawField : public RawObject {
   RAW_HEAP_OBJECT_IMPLEMENTATION(Field);
 
   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.
     RawSmi* offset_;  // Offset in words for instance fields.
   } value_;
   union {
     // When precompiling we need to save the static initializer function here
     // so that code for it can be generated.
     RawFunction* precompiled_;  // Static initializer function - precompiling.
     // When generating script snapshots after running the application it is
@@ skipping 1456 matching lines @@
   COMPILE_ASSERT(kExternalTypedDataInt8ArrayCid ==
                  kTypedDataInt8ArrayViewCid + 15);
   COMPILE_ASSERT(kByteBufferCid == kExternalTypedDataInt8ArrayCid + 14);
   COMPILE_ASSERT(kNullCid == kByteBufferCid + 1);
   return (kNullCid - kTypedDataInt8ArrayCid);
 }
 
 }  // namespace dart
 
 #endif  // VM_RAW_OBJECT_H_