pkg/pool: remove usage of Chain.track

lib/pool.dart

Index: lib/pool.dart
diff --git a/lib/pool.dart b/lib/pool.dart
index 6941229b3e084d18f2d2096ed0630569b688f9bb..e8ee99cab190783c9f0b9e97a89720c165fa5637 100644
--- a/lib/pool.dart
+++ b/lib/pool.dart
@@ -87,8 +87,7 @@ class Pool {
   ///
   /// 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.