Don't warn if a transformer declares outputs that it doesn't emit.

pkg/barback/lib/src/transform_node.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.transform_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 393 matching lines @@
     var invalidIds = newOutputs
         .map((asset) => asset.id)
         .where((id) => id.package != phase.cascade.package)
         .toSet();
     for (var id in invalidIds) {
       newOutputs.removeId(id);
       // TODO(nweiz): report this as a warning rather than a failing error.
       phase.cascade.reportError(new InvalidOutputException(info, id));
     }
 
-    if (_declaredOutputs != null) {
-      var missingOutputs = _declaredOutputs.difference(
-          newOutputs.map((asset) => asset.id).toSet());
-      if (missingOutputs.isNotEmpty) {
-        _warn("This transformer didn't emit declared "
-            "${pluralize('output asset', missingOutputs.length)} "
-            "${toSentence(missingOutputs)}.");
-      }
-    }
-
     // Remove outputs that used to exist but don't anymore.
     for (var id in _outputControllers.keys.toList()) {
       if (newOutputs.containsId(id)) continue;
       _outputControllers.remove(id).setRemoved();
     }
 
     // Emit or stop emitting the pass-through asset between removing and
     // adding outputs to ensure there are no collisions.
     if (!_consumePrimary && !newOutputs.containsId(primary.id)) {
       _emitPassThrough();
@@ skipping 122 matching lines @@
   ///
   /// This will never transition to another state.
   static final NOT_PRIMARY = const _State._("not primary");
 
   final String name;
 
   const _State._(this.name);
 
   String toString() => name;
 }