Consistently use setters to set event handlers in native APIs.

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 128 matching lines @@
     if (_closed) {
       throw new ProcessException("Process closed");
     }
     _in.close();
     _out.close();
     _err.close();
     _exitHandler.close();
     _closed = true;
   }
 
-  void setExitHandler(void callback(int exitCode)) {
+  void set exitHandler(void callback(int exitCode)) {
     if (_closed) {
       throw new ProcessException("Process closed");
     }
     if (_killed) {
       throw new ProcessException("Process killed");
     }
     _exitHandlerCallback = callback;
   }
 
   String _path;
   ObjectArray<String> _arguments;
   Socket _in;
   Socket _out;
   Socket _err;
   Socket _exitHandler;
   int _pid;
   bool _closed;
   bool _killed;
   bool _started;
   var _exitHandlerCallback;
 }