Reorganize barback's source files.

pkg/barback/lib/src/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;
-
-/// 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.
-  ///
-  /// The contents of the map should be serializable across isolates, but
-  /// otherwise can contain whatever you want.
-  final Map configuration;
-
-  /// The mode that user is running Barback in.
-  ///
-  /// This will be the same for all transformers in a running instance of
-  /// Barback.
-  final BarbackMode mode;
-
-  BarbackSettings(this.configuration, this.mode);
-}
-
-/// Enum-like class for specifying a mode that transformers may be run in.
-///
-/// Note that this is not a *closed* set of enum values. Host applications may
-/// define their own values for this, so a transformer relying on it should
-/// ensure that it behaves sanely with unknown values.
-class BarbackMode {
-  /// The normal mode used during development.
-  static const DEBUG = const BarbackMode._("debug");
-
-  /// The normal mode used to build an application for deploying to production.
-  static const RELEASE = const BarbackMode._("release");
-
-  /// The name of the mode.
-  ///
-  /// By convention, this is a lowercase string.
-  final String name;
-
-  /// Create a mode named [name].
-  factory BarbackMode(String name) {
-    // Use canonical instances of known names.
-    switch (name) {
-      case "debug": return BarbackMode.DEBUG;
-      case "release": return BarbackMode.RELEASE;
-      default:
-        return new BarbackMode._(name);
-    }
-  }
-
-  const BarbackMode._(this.name);
-
-  String toString() => name;
-}