pkg/pool: remove usage of Chain.track

test/pool_test.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.
 
 import 'dart:async';
 
 import 'package:fake_async/fake_async.dart';
 import 'package:pool/pool.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:test/test.dart';
@@ skipping 145 matching lines @@
         expect(pool.request(), throwsA(new isInstanceOf<TimeoutException>()));
 
         async.elapse(new Duration(seconds: 6));
       });
     });
   });
 
   group("allowRelease()", () {
     test("runs the callback once the resource limit is exceeded", () async {
       var pool = new Pool(50);
-      var requests = [];
       for (var i = 0; i < 49; i++) {
         expect(pool.request(), completes);
       }
 
       var resource = await pool.request();
       var onReleaseCalled = false;
       resource.allowRelease(() => onReleaseCalled = true);
       await new Future.delayed(Duration.ZERO);
       expect(onReleaseCalled, isFalse);
 
@@ skipping 110 matching lines @@
 ///
 /// This should only be called within a [FakeAsync.run] zone.
 Matcher get doesNotComplete => predicate((future) {
   expect(future, new isInstanceOf<Future>());
 
   var stack = new Trace.current(1);
   future.then((_) => registerException(
       new TestFailure("Expected future not to complete."), stack));
   return true;
 });