Fix process test and implementation after change to setter for registering event handler.

tests/standalone/src/ProcessStartExceptionTest.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.
 //
 // Process test program to test process communication.
 
 class ProcessStartExceptionTest {
 
   static void testStartException() {
     Process process =
         new Process("__path_to_something_that_hopefully_does_not_exist__",
                     const []);
 
     void exitHandler(int exitCode) {
       exitHandlerCalled = false;
     }
 
-    process.setExitHandler(exitHandler);
+    process.exitHandler = exitHandler;
     try {
       process.start();
     } catch (ProcessException e) {
       errorCode = e.errorCode;
     }
   }
 
   static void testMain() {
     exitHandlerCalled = false;
     testStartException();
     Expect.equals(2, errorCode);
     Expect.equals(false, exitHandlerCalled);
   }
 
   static int errorCode;
   static bool exitHandlerCalled;
 }
 
 main() {
   ProcessStartExceptionTest.testMain();
 }