Fix a few warnings.

pkg/barback/lib/src/phase.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.phase;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset_cascade.dart';
@@ skipping 147 matching lines @@
   /// This will remove all the phase's outputs and all following phases.
   void remove() {
     removeFollowing();
     for (var input in _inputs.values.toList()) {
       input.remove();
     }
     _onDirtyPool.close();
   }
 
   /// Remove all phases after this one.
-  Phase removeFollowing() {
+  void removeFollowing() {
     if (_next == null) return;
     _next.remove();
     _next = null;
   }
 
   /// Processes this phase.
   ///
   /// Returns a future that completes when processing is done. If there is
   /// nothing to process, returns `null`.
   Future process() {
@@ skipping 22 matching lines @@
       for (var id in outputIds) {
         // It's possible the output was removed before other transforms in this
         // phase finished.
         if (!_outputs.containsKey(id)) continue;
         var exception = _outputs[id].collisionException;
         if (exception != null) cascade.reportError(exception);
       }
     });
   }
 }