Remove fields from Isolate in Product mode

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index 42e16bb5e9673eac9443a229cda777485d7e370b..4553d1b5f9e483e93de952100bcda410fe1d6e92 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -155,7 +155,7 @@ char* Dart::InitOnce(const uint8_t* vm_isolate_snapshot,
     Profiler::InitOnce();
   }
   SemiSpace::InitOnce();
-  Metric::InitOnce();
+  NOT_IN_PRODUCT(Metric::InitOnce());
   StoreBuffer::InitOnce();
   MarkingStack::InitOnce();
 
@@ -372,7 +372,7 @@ const char* Dart::Cleanup() {
     }
     bool result = Thread::EnterIsolate(vm_isolate_);
     ASSERT(result);
-    Metric::Cleanup();
+    NOT_IN_PRODUCT(Metric::Cleanup());
     Thread::ExitIsolate();
   }