Add --dump-megamorphic-stats. Lower megamorphic cache load factor to 50%.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index c6c61f5489af018ec0fcf5d1082aa9920dcab8c5..b184f6cce2a3bbfb3ceff21de569e12dabc6a94f 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1608,9 +1608,11 @@ void Isolate::LowLevelShutdown() {
   FinalizeWeakPersistentHandlesVisitor visitor;
   api_state()->weak_persistent_handles().VisitHandles(&visitor);
 
+  if (FLAG_dump_megamorphic_stats) {
+    MegamorphicCacheTable::PrintSizes(this);
+  }
   if (FLAG_trace_isolates) {
     heap()->PrintSizes();
-    MegamorphicCacheTable::PrintSizes(this);
     Symbols::DumpStats();
     OS::Print("[-] Stopping isolate:\n"
               "\tisolate:    %s\n", name());