Fix some minor static type issues in dart:io implementation.

runtime/bin/process_impl.dart

 // 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.
 
 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 179 matching lines @@
     _onError = callback;
   }
 
   void set onStart(void callback()) {
     _onStart = callback;
   }
 
   String _path;
   ObjectArray<String> _arguments;
   String _workingDirectory;
-  Socket _in;
-  Socket _out;
-  Socket _err;
+  // Private methods of _Socket are used by _in, _out, and _err.
+  _Socket _in;
+  _Socket _out;
+  _Socket _err;
   Socket _exitHandler;
   int _pid;
   bool _closed;
   bool _killed;
   bool _started;
   Function _onExit;
   Function _onError;
   Function _onStart;
 }