Rename blahHandler to onBlah throughout dart:io.

tests/standalone/src/ProcessBrokenPipeTest.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.
 //
 // Process test program to test closed stdin from child process.
 
 #import('dart:io');
 
 #source("ProcessTestUtil.dart");
 
 main() {
   // Running dart without arguments makes it close right away.
   Process process = new Process.start(getDartFileName(), []);
 
   // Write to the stdin after the process is terminated to test
   // writing to a broken pipe.
-  process.exitHandler = (code) {
+  process.onExit = (code) {
     Expect.isFalse(process.stdin.write([0]));
     process.close();
   };
 }