Rename blahHandler to onBlah throughout dart:io.

tests/standalone/src/ProcessExitNegativeTest.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 that compilation errors in the process
 // exit handler is reported correctly.
 
 #import("dart:io");
 #source("ProcessTestUtil.dart");
 
 void main() {
   Process p = new Process.start(getProcessTestFileName(),
                                 const ["0", "0", "0", "0"]);
-  p.exitHandler = (int s) {
+  p.onExit = (int s) {
     print(a.toString());  // Should cause a compilation error here.
     p.close();
   };
 }