Silence service isolate startup error unless --trace-service is enabled

runtime/vm/service_isolate.cc

Index: runtime/vm/service_isolate.cc
diff --git a/runtime/vm/service_isolate.cc b/runtime/vm/service_isolate.cc
index 19ccf685da512528ab8a910740a524d7ae549fb2..c5e2e753039753048003e07879311161ffbde059 100644
--- a/runtime/vm/service_isolate.cc
+++ b/runtime/vm/service_isolate.cc
@@ -331,7 +331,9 @@ class RunServiceTask : public ThreadPool::Task {
                                                    NULL,
                                                    &error));
     if (isolate == NULL) {
-      OS::PrintErr("vm-service: Isolate creation error: %s\n", error);
+      if (FLAG_trace_service) {
+        OS::PrintErr("vm-service: Isolate creation error: %s\n", error);
+      }
       ServiceIsolate::SetServiceIsolate(NULL);
       ServiceIsolate::FinishedInitializing();
       ServiceIsolate::FinishedExiting();