Remove fields from Isolate in Product mode

runtime/vm/metrics.cc

 // Copyright (c) 2014, 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.
 
+#if !defined(PRODUCT)
+
 #include "vm/metrics.h"
 
 #include "vm/isolate.h"
 #include "vm/json_stream.h"
 #include "vm/log.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
 #include "vm/runtime_entry.h"
 
 namespace dart {
@@ skipping 316 matching lines @@
   set_value(kMaxInt64);
 }
 
 void MinMetric::SetValue(int64_t new_value) {
   if (new_value < value()) {
     set_value(new_value);
   }
 }
 
 }  // namespace dart
+
+#endif  // !defined(PRODUCT)