Fix all strong-mode warnings.

lib/src/graph/group_runner.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.graph.group_runner;
 
 import 'dart:async';
 
 import '../asset/asset_node.dart';
 import '../log.dart';
@@ skipping 17 matching lines @@
 
   /// How far along [this] is in processing its assets.
   NodeStatus get status {
     // Just check the last phase, since it will check all the previous phases
     // itself.
     return _phases.last.status;
   }
 
   /// A stream that emits an event every time the group's status changes.
   Stream<NodeStatus> get onStatusChange => _onStatusChange;
-  Stream _onStatusChange;
+  Stream<NodeStatus> _onStatusChange;
 
   /// A stream that emits any new assets emitted by [this].
   ///
   /// Assets are emitted synchronously to ensure that any changes are thoroughly
   /// propagated as soon as they occur.
   Stream<AssetNode> get onAsset => _onAsset;
   Stream<AssetNode> _onAsset;
 
   /// A stream that emits an event whenever any transforms in this group logs
   /// an entry.
@@ skipping 31 matching lines @@
   /// Removes this group and all sub-phases within it.
   void remove() {
     _onLogPool.close();
     for (var phase in _phases) {
       phase.remove();
     }
   }
 
   String toString() => "group in phase $_location for $_group";
 }