Fix strong mode warnings.

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 88 matching lines @@
       _requestedResources.add(completer);
       _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()) {
+  Future/*<T>*/ withResource/*<T>*/(/*=T*/ callback()) async {
     if (isClosed) {
       throw new StateError(
           "withResource() may not be called on a closed Pool.");
     }
 
-    // TODO(nweiz): Use async/await when sdk#23497 is fixed.
-    return request().then((resource) {
-      return new Future.sync(callback).whenComplete(resource.release);
-    });
+    var resource = await request();
+    try {
+      return await callback();
+    } finally {
+      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
   /// returned future completes successfully. If any `onRelease` callback throws
@@ skipping 53 matching lines @@
   ///
   /// Futures returned by [_runOnRelease] always complete in the order they were
   /// created, even if earlier [onRelease] callbacks take longer to run.
   Future<PoolResource> _runOnRelease(onRelease()) {
     new Future.sync(onRelease).then((value) {
       _onReleaseCompleters.removeFirst().complete(new PoolResource._(this));
     }).catchError((error, stackTrace) {
       _onReleaseCompleters.removeFirst().completeError(error, stackTrace);
     });
 
-    var completer = new Completer.sync();
+    var completer = new Completer<PoolResource>.sync();
     _onReleaseCompleters.add(completer);
     return completer.future;
   }
 
   /// A resource has been requested, allocated, or released.
   void _resetTimer() {
     if (_timer == null) return;
 
     if (_requestedResources.isEmpty) {
       _timer.cancel();
@@ skipping 52 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);
   }
 }