Fix typo.

sdk/lib/async/zone.dart

 // Copyright (c) 2013, 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.
 
 part of dart.async;
 
 typedef dynamic ZoneCallback();
 typedef dynamic ZoneUnaryCallback(arg);
 typedef dynamic ZoneBinaryCallback(arg1, arg2);
 
@@ skipping 504 matching lines @@
   Timer createPeriodicTimer(Zone zone, Duration period, void f(Timer timer)) {
     _BaseZone parent = _degelationTarget;
     while (parent._specification.createPeriodicTimer == null) {
       parent = parent.parent;
     }
     return (parent._specification.createPeriodicTimer)(
         parent, new _ZoneDelegate(parent.parent), zone, period, f);
   }
 
   void print(Zone zone, String line) {
-    _CustomizedZone parent = _degelationTarget;
+    _BaseZone parent = _degelationTarget;
     while (parent._specification.print == null) {
       parent = parent.parent;
     }
     (parent._specification.print)(
         parent, new _ZoneDelegate(parent.parent), zone, line);
   }
 
   Zone fork(Zone zone, ZoneSpecification specification,
             Map<Symbol, dynamic> zoneValues) {
     _BaseZone parent = _degelationTarget;
@@ skipping 475 matching lines @@
   ZoneSpecification specification =
     new ZoneSpecification(handleUncaughtError: errorHandler,
                           scheduleMicrotask: asyncHandler);
   Zone zone = Zone.current.fork(specification: specification);
   if (onError != null) {
     return zone.runGuarded(body);
   } else {
     return zone.run(body);
   }
 }