Don't execute VmStats::Stop() unless port is specified.

runtime/bin/main.cc

 // Copyright (c) 2012, 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 <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 
 #include "include/dart_api.h"
 #include "include/dart_debugger_api.h"
@@ skipping 801 matching lines @@
       return ErrorExit("%s\n", Dart_GetError(result));
     }
     // Keep handling messages until the last active receive port is closed.
     result = Dart_RunLoop();
     if (Dart_IsError(result)) {
       return ErrorExit("%s\n", Dart_GetError(result));
     }
   }
 
   Dart_ExitScope();
-  VmStats::Stop();
+  if (vmstats_port >= 0) {
+    VmStats::Stop();
+  }
   // Shutdown the isolate.
   Dart_ShutdownIsolate();
   // Terminate process exit-code handler.
   Process::TerminateExitCodeHandler();
   // Free copied argument strings if converted.
   if (argv_converted) {
     for (int i = 0; i < argc; i++) free(argv[i]);
   }
 
   return Process::GlobalExitCode();
 }