Let Dart code running in Sky add events to the trace timeline

sky/engine/core/script/dart_controller.cc

Index: sky/engine/core/script/dart_controller.cc
diff --git a/sky/engine/core/script/dart_controller.cc b/sky/engine/core/script/dart_controller.cc
index 270ed4e0eda99bdeeb90012cb08dddf03977b431..3fa1fbb45a7f47daf20112fc844e26faaedb3c9a 100644
--- a/sky/engine/core/script/dart_controller.cc
+++ b/sky/engine/core/script/dart_controller.cc
@@ -212,6 +212,8 @@ static Dart_Isolate IsolateCreateCallback(const char* script_uri,
 }
 
 static void CallHandleMessage(base::WeakPtr<DartState> dart_state) {
+  TRACE_EVENT0("sky", "CallHandleMessage");
+
   if (!dart_state)
     return;