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

runtime/vm/service_isolate.cc

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/service_isolate.h"
 
 #include "vm/compiler.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/isolate.h"
@@ skipping 313 matching lines @@
 
     isolate =
         reinterpret_cast<Isolate*>(create_callback(ServiceIsolate::kName,
                                                    NULL,
                                                    NULL,
                                                    NULL,
                                                    &api_flags,
                                                    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();
       return;
     }
 
     bool got_unwind;
     {
       ASSERT(Isolate::Current() == NULL);
       StartIsolateScope start_scope(isolate);
@@ skipping 178 matching lines @@
   }
   ASSERT(port != ILLEGAL_PORT);
   ServiceIsolate::SetServicePort(port);
 }
 
 
 void ServiceIsolate::VisitObjectPointers(ObjectPointerVisitor* visitor) {
 }
 
 }  // namespace dart