Add missing import of stack_trace to barback implementation.

pkg/barback/lib/src/pool.dart

 // Copyright (c) 2013, 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;
 
 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
@@ skipping 109 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();
   }
-}
+}