pkg/pool: remove usage of Chain.track

lib/pool.dart

 // Copyright (c) 2014, 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.
 
 library pool;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 69 matching lines @@
       _resetTimer();
       return completer.future;
     }
   }
 
   /// Requests a resource for the duration of [callback], which may return a
   /// Future.
   ///
   /// The return value of [callback] is piped to the returned Future.
   Future withResource(callback()) {
-    return request().then((resource) =>
-        Chain.track(new Future.sync(callback)).whenComplete(resource.release));
+    return request().then((resource) => new Future.sync(callback).whenComplete(resource.release));
   }
 
   /// If there are any pending requests, this will fire the oldest one.
   void _onResourceReleased() {
     _resetTimer();
 
     if (_requestedResources.isEmpty) {
       _allocatedResources--;
       return;
     }
@@ skipping 95 matching lines @@
   /// may be complete, but it could still emit asynchronous errors.
   void allowRelease(onRelease()) {
     if (_released) {
       throw new StateError("A PoolResource may only be released once.");
     }
     _released = true;
     _pool._onResourceReleaseAllowed(onRelease);
   }
 }