Fix a new strong-mode error.

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.
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:async/async.dart';
 import 'package:stack_trace/stack_trace.dart';
 
@@ skipping 98 matching lines @@
   Future/*<T>*/ withResource/*<T>*/(/*=T*/ callback()) {
     if (isClosed) {
       throw new StateError(
           "withResource() may not be called on a closed Pool.");
     }
 
     // We can't use async/await here because we need to start the request
     // synchronously in case the pool is closed immediately afterwards. Async
     // functions have an asynchronous gap between calling and running the body,
     // and [close] could be called during that gap. See #3.
-    return request().then((resource) {
-      return new Future.sync(callback).whenComplete(resource.release);
+    return request().then/*<Future<T>>*/((resource) {
+      return new Future/*<T>*/.sync(callback).whenComplete(resource.release);
     });
   }
 
   /// Closes the pool so that no more resources are requested.
   ///
   /// Existing resource requests remain unchanged.
   ///
   /// Any resources that are marked as releasable using
   /// [PoolResource.allowRelease] are released immediately. Once all resources
   /// have been released and any `onRelease` callbacks have completed, the
@@ skipping 127 matching lines @@
   /// produce additional information later on. For example, an isolate's task
   /// 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);
   }
 }