Add missing import of stack_trace to barback implementation.

pkg/barback/lib/src/pool.dart

Index: pkg/barback/lib/src/pool.dart
diff --git a/pkg/barback/lib/src/pool.dart b/pkg/barback/lib/src/pool.dart
index c50d19118247dd9ec69735e6eb3df8b37adf8304..4653722528ad5cefdac2eb472f0e5cddd1c62f70 100644
--- a/pkg/barback/lib/src/pool.dart
+++ b/pkg/barback/lib/src/pool.dart
@@ -7,6 +7,8 @@ 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.
@@ -136,4 +138,4 @@ class PoolResource {
     _released = true;
     _pool._onResourceReleased();
   }
-}
+}