Fix product build.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index bb8d4469144f63d1a952a38e2f59c1160a9bc09f..2c92b17c52f3692024ff02d1c4f8fd54507c8123 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -837,6 +837,7 @@ void Heap::PrintStats() {
 
 
 void Heap::PrintStatsToTimeline(TimelineEventScope* event) {
+#if !defined(PRODUCT)
   if ((event == NULL) || !event->enabled()) {
     return;
   }
@@ -891,6 +892,7 @@ void Heap::PrintStatsToTimeline(TimelineEventScope* event) {
                         "After.Old.External (kB)",
                         "%" Pd "",
                         RoundWordsToKB(stats_.after_.old_.external_in_words));
+#endif  // !defined(PRODUCT)
 }