Undo change that converted exit code 255 to -1.

runtime/bin/process_macos.cc

 // Copyright (c) 2011, 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/process.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <stdio.h>
@@ skipping 74 matching lines @@
 
 
 void ExitHandler(int process_signal, siginfo_t* siginfo, void* tmp) {
   int pid = 0;
   int status = 0;
   while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
     int exit_code = 0;
     int negative = 0;
     if (WIFEXITED(status)) {
       exit_code = WEXITSTATUS(status);
-      if (exit_code == 255) {
-        exit_code = 1;
-        negative = 1;
-      }
     }
     if (WIFSIGNALED(status)) {
       exit_code = WTERMSIG(status);
       negative = 1;
     }
     ProcessInfo* process = LookupProcess(pid);
     if (process != NULL) {
       int message[3] = { pid, exit_code, negative };
       intptr_t result =
           FDUtils::WriteToBlocking(process->fd(), &message, sizeof(message));
@@ skipping 240 matching lines @@
   if (result == -1) {
     return false;
   }
   return true;
 }
 
 
 void Process::Exit(intptr_t id) {
   RemoveProcess(id);
 }