Rename blahHandler to onBlah throughout dart:io.

tests/standalone/src/ProcessExitTest.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 //
 // Process test program to test process communication.
 
 #library("ProcessExitTest");
 #import("dart:io");
 #source("ProcessTestUtil.dart");
 
 class ProcessExitTest {
   static void testExit() {
     Process process = new Process.start(getProcessTestFileName(),
                                         const ["0", "0", "99", "0"]);
 
-    process.exitHandler = (int exitCode) {
+    process.onExit = (int exitCode) {
       Expect.equals(exitCode, 99);
       process.close();
     };
   }
 }
 
 main() {
   ProcessExitTest.testExit();
 }