Fix an incorrect method invocation in barback.

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 23b878f0c548c0270346fcc2a8aca0a96adb0b2f..7f89c12821ec36012b94f54d0e16e167c8a85d50 100644
--- a/pkg/barback/lib/src/pool.dart
+++ b/pkg/barback/lib/src/pool.dart
@@ -106,7 +106,7 @@ class Pool {
   /// emit exceptions.
   void _onTimeout() {
     for (var completer in _requestedResources) {
-      completer.completeException("Pool deadlock: all resources have been "
+      completer.completeError("Pool deadlock: all resources have been "
           "allocated for too long.", new Trace.current().vmTrace);
     }
     _requestedResources.clear();