VM: Refactor allocation stats code and remove duplicate code.

runtime/vm/scavenger.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_SCAVENGER_H_
 #define VM_SCAVENGER_H_
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/dart.h"
@@ skipping 193 matching lines @@
   }
 
   void IncrementCollections() {
     collections_++;
   }
 
   intptr_t collections() const {
     return collections_;
   }
 
-  void PrintToJSONObject(JSONObject* object);
+  void PrintToJSONObject(JSONObject* object) const;
 
   void AllocateExternal(intptr_t size);
   void FreeExternal(intptr_t size);
 
  private:
   // Ids for time and data records in Heap::GCStats.
   enum {
     // Time
     kVisitIsolateRoots = 0,
     kIterateStoreBuffers = 1,
@@ skipping 87 matching lines @@
 
   friend class ScavengerVisitor;
   friend class ScavengerWeakVisitor;
 
   DISALLOW_COPY_AND_ASSIGN(Scavenger);
 };
 
 }  // namespace dart
 
 #endif  // VM_SCAVENGER_H_