Correct the sign of exit codes for the process library.

runtime/bin/process_impl.dart

 // 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.
 
 class _ProcessStartStatus {
   int _errorCode;  // Set to OS error code if process start failed.
   String _errorMessage;  // Set to OS error message if process start failed.
 }
 
 
@@ skipping 21 matching lines @@
     _in = new _Socket._internalOutputOnly();
     _out = new _Socket._internalInputOnly();
     _err = new _Socket._internalOutputOnly();
     _exitHandler = new _Socket._internal();
     _closed = false;
     _killed = false;
     _started = false;
     _exitHandlerCallback = null;
   }
 
+  int _intFromBytes(List<int> bytes, int offset) {
+    return (bytes[offset] +
+            (bytes[offset + 1] << 8) +
+            (bytes[offset + 2] << 16) +
+            (bytes[offset + 3] << 24));
+  }
+
   void start() {
     var status = new _ProcessStartStatus();
     bool success = _start(
         _path, _arguments, _in, _out, _err, _exitHandler, status);
     if (!success) {
       close();
       throw new ProcessException(status._errorMessage, status._errorCode);
     }
     _started = true;
 
     // Make sure to activate socket handlers now that the file
     // descriptors have been set.
     _in._activateHandlers();
     _out._activateHandlers();
     _err._activateHandlers();
 
     // Setup an exit handler to handle internal cleanup and possible
     // callback when a process terminates.
     _exitHandler.dataHandler = () {
-        final int EXIT_DATA_SIZE = 8;
+        final int EXIT_DATA_SIZE = 12;
         List<int> exitDataBuffer = new List<int>(EXIT_DATA_SIZE);
         InputStream input = _exitHandler.inputStream;
         int exitDataRead = 0;
 
         int exitCode(List<int> ints) {
-          return ints[4] + (ints[5] << 8) + (ints[6] << 16) + (ints[7] << 24);
+          var code = _intFromBytes(ints, 4);
+          var negative = _intFromBytes(ints, 8);
+          assert(negative == 0 || negative == 1);
+          return (negative == 0) ? code : -code;
         }
 
         int exitPid(List<int> ints) {
-          return ints[0] + (ints[1] << 8) + (ints[2] << 16) + (ints[3] << 24);
+          return _intFromBytes(ints, 0);
         }
 
         void handleExit() {
           _processExit(exitPid(exitDataBuffer));
           if (_exitHandlerCallback !== null) {
             _exitHandlerCallback(exitCode(exitDataBuffer));
           }
         }
 
         void exitData() {
@@ skipping 79 matching lines @@
   Socket _in;
   Socket _out;
   Socket _err;
   Socket _exitHandler;
   int _pid;
   bool _closed;
   bool _killed;
   bool _started;
   var _exitHandlerCallback;
 }