Use native fields for process pid and fix bug that allowed killing

runtime/bin/process.h

 // 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.
 
 #ifndef BIN_PROCESS_H_
 #define BIN_PROCESS_H_
 
 #include "bin/builtin.h"
 #include "platform/globals.h"
 
@@ skipping 18 matching lines @@
 
   // Kill a process with a given pid.
   static bool Kill(intptr_t id, int signal);
 
   // Terminate the exit code handler thread. Does not return before
   // the thread has terminated.
   static void TerminateExitCodeHandler();
 
   static intptr_t CurrentProcessId();
 
+  static Dart_Handle GetProcessIdNativeField(Dart_Handle process,
+                                             intptr_t* pid);
+  static Dart_Handle SetProcessIdNativeField(Dart_Handle process,
+                                             intptr_t pid);
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Process);
 };
 
 #endif  // BIN_PROCESS_H_