Fix deadlock in process exit handling

runtime/bin/process.h

 // 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.
 
 #ifndef BIN_PROCESS_H_
 #define BIN_PROCESS_H_
 
 #include "bin/builtin.h"
 #include "bin/globals.h"
 
 
 class Process {
  public:
+  // Start a new process providing access to stdin, stdout, stderr and
+  // process exit streams.
   static int Start(const char* path,
                    char* arguments[],
                    intptr_t arguments_length,
                    intptr_t* in,
                    intptr_t* out,
                    intptr_t* err,
                    intptr_t* id,
                    intptr_t* exit_handler,
                    char* os_error_message,
                    int os_error_message_len);
 
+  // Kill a process with a given pid.
   static bool Kill(intptr_t id);
 
+  // Indicate that the process with the given pid has exited.
+  static void Exit(intptr_t id);
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Process);
 };
 
 #endif  // BIN_PROCESS_H_