Dump information when we hit a FATAL in become

runtime/vm/become.h

 // Copyright (c) 2016, 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_BECOME_H_
 #define VM_BECOME_H_
 
 #include "vm/allocation.h"
 #include "vm/raw_object.h"
 
@@ skipping 67 matching lines @@
   // Smalltalk's one-way bulk become (Array>>#elementsForwardIdentityTo:).
   // Redirects all pointers to elements of 'before' to the corresponding element
   // in 'after'. Every element in 'before' is guaranteed to be not reachable.
   // Useful for atomically applying behavior and schema changes.
   static void ElementsForwardIdentity(const Array& before, const Array& after);
 
   // Convert and instance object into a dummy object,
   // making the instance independent of its class.
   // (used for morphic instances during reload).
   static void MakeDummyObject(const Instance& instance);
+
+ private:
+  static void CrashDump(RawObject* before_obj, RawObject* after_obj);
 };
 
 }  // namespace dart
 
 #endif  // VM_BECOME_H_