Move pub/barback's Pool class into its own package.

pkg/pool/lib/pool.dart

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// 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 barback.pool;
+library pool;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:stack_trace/stack_trace.dart';
 
-import 'utils.dart';
-
-// TODO(nweiz): put this somewhere that it can be shared between packages.
 /// Manages an abstract pool of resources with a limit on how many may be in use
 /// at once.
 ///
 /// When a resource is needed, the user should call [request]. When the returned
 /// future completes with a [PoolResource], the resource may be allocated. Once
 /// the resource has been released, the user should call [PoolResource.release].
 /// The pool will ensure that only a certain number of [PoolResource]s may be
 /// allocated at once.
 class Pool {
   /// Completers for requests beyond the first [_maxAllocatedResources].
@@ skipping 44 matching lines @@
       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) =>
-        syncFuture(callback).whenComplete(resource.release));
+        Chain.track(new Future.sync(callback)).whenComplete(resource.release));
   }
 
   /// If there are any pending requests, this will fire the oldest one.
   void _onResourceReleased() {
     if (_requestedResources.isEmpty) {
       _allocatedResources--;
       if (_timer != null) {
         _timer.cancel();
         _timer = null;
       }
@@ skipping 41 matching lines @@
 
   /// Tells the parent [Pool] that the resource associated with this resource is
   /// no longer allocated, and that a new [PoolResource] may be allocated.
   void release() {
     if (_released) {
       throw new StateError("A PoolResource may only be released once.");
     }
     _released = true;
     _pool._onResourceReleased();
   }
-}
+}
+