Rename blahHandler to onBlah throughout dart:io.

tests/standalone/src/ProcessStartExceptionTest.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.
 
 #import("dart:io");
 
 class ProcessStartExceptionTest {
 
   static void testStartError() {
     Process process =
         new Process.start("__path_to_something_that_should_not_exist__",
                           const []);
 
-    process.exitHandler = (int exitCode) {
+    process.onExit = (int exitCode) {
       Expect.fail("exit handler called");
     };
 
-    process.errorHandler = (ProcessException e) {
+    process.onError = (ProcessException e) {
       Expect.equals(2, e.errorCode);
     };
   }
 }
 
 main() {
   ProcessStartExceptionTest.testStartError();
 }