Remove the debugger_ field from Isolate in a PRODUCT build

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index 43ba92e1f1379bbd825ccb9df8eda352241c4265..ae978baa87ffe13fae890096fc333aa8bbe02d4c 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -617,9 +617,12 @@ RawError* Dart::InitializeIsolate(const uint8_t* snapshot_data,
   }
 
   ServiceIsolate::SendIsolateStartupMessage();
+#if !defined(PRODUCT)
   if (FLAG_support_debugger) {
     I->debugger()->NotifyIsolateCreated();
   }
+#endif
+
   // Create tag table.
   I->set_tag_table(GrowableObjectArray::Handle(GrowableObjectArray::New()));
   // Set up default UserTag.