Reorganize barback's source files.

pkg/barback/lib/src/transformer/barback_settings.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.barback_settings;
+library barback.transformer.barback_settings;
 
 /// A generic settings object for providing configuration details to
 /// [Transformer]s.
 ///
 /// Barback does not specify *how* this is provided to transformers. It is up
 /// to a host application to handle this. (For example, pub passes this to the
 /// transformer's constructor.)
 class BarbackSettings {
   /// An open-ended map of configuration properties specific to this
   /// transformer.
@@ skipping 36 matching lines @@
       case "release": return BarbackMode.RELEASE;
       default:
         return new BarbackMode._(name);
     }
   }
 
   const BarbackMode._(this.name);
 
   String toString() => name;
 }