Remove fields from Isolate in Product mode

runtime/vm/metrics.cc

Index: runtime/vm/metrics.cc
diff --git a/runtime/vm/metrics.cc b/runtime/vm/metrics.cc
index 5489823047c4f7f9233ab74d5f095661df7c03d9..afb7b879bc9776860101d6464037e4327f9de6a3 100644
--- a/runtime/vm/metrics.cc
+++ b/runtime/vm/metrics.cc
@@ -2,6 +2,8 @@
 // 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"
@@ -338,3 +340,5 @@ void MinMetric::SetValue(int64_t new_value) {
 }
 
 }  // namespace dart
+
+#endif  // !defined(PRODUCT)