Use a named constant for the object id size when writing out the header.

runtime/vm/heap_profiler.cc

 // 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.
 
 #include "vm/heap_profiler.h"
 
 #include "vm/dart_api_state.h"
 #include "vm/object.h"
 #include "vm/raw_object.h"
 #include "vm/stack_frame.h"
@@ skipping 287 matching lines @@
 // Header
 //
 // Format:
 //   [u1]* - format name
 //   u4 - size of identifiers
 //   u4 - high word of number of milliseconds since 0:00 GMT, 1/1/70
 //   u4 - low word of number of milliseconds since 0:00 GMT, 1/1/70
 void HeapProfiler::WriteHeader() {
   const char magic[] = "JAVA PROFILE 1.0.1";
   Write(magic, sizeof(magic));
-  uint32_t size = htonl(8);
+  uint32_t size = htonl(kObjectIdSize);
   Write(&size, sizeof(size));
   uint64_t milliseconds = OS::GetCurrentTimeMillis();
   uint32_t hi = htonl((uint32_t)((milliseconds >> 32) & 0x00000000FFFFFFFF));
   Write(&hi, sizeof(hi));
   uint32_t lo = htonl((uint32_t)(milliseconds & 0x00000000FFFFFFFF));
   Write(&lo, sizeof(lo));
 }
 
 
 // Record
@@ skipping 396 matching lines @@
   RawObject* raw_obj = handle->raw();
   visitor_->VisitPointer(&raw_obj);
 }
 
 
 void HeapProfilerObjectVisitor::VisitObject(RawObject* raw_obj) {
   profiler_->WriteObject(raw_obj);
 }
 
 }  // namespace dart