[dart:io] Adds a finalizer to _NativeSocket to avoid socket leaks

runtime/bin/error_exit.cc

 // Copyright (c) 2017, 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 "bin/error_exit.h"
 
 #include "bin/log.h"
 #include "bin/platform.h"
 #include "bin/process.h"
 #include "bin/eventhandler.h"
@@ skipping 15 matching lines @@
 
   // Terminate process exit-code handler.
   Process::TerminateExitCodeHandler();
 
   char* error = Dart_Cleanup();
   if (error != NULL) {
     Log::PrintErr("VM cleanup failed: %s\n", error);
     free(error);
   }
 
+  Process::ClearAllSignalHandlers();
   EventHandler::Stop();
   Platform::Exit(exit_code);
 }
 
 }  // namespace bin
 }  // namespace dart