Refactor Source and SourceRegistry.

lib/src/barback/asset_environment.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
 import 'package:watcher/watcher.dart';
@@ skipping 591 matching lines @@
 
   /// Adds a file watcher for [dir] within [package], if the directory exists
   /// and the package needs watching.
   Future<StreamSubscription<WatchEvent>> _watchDirectorySources(
       Package package, String dir) {
     // If this package comes from a cached source, its contents won't change so
     // we don't need to monitor it. `packageId` will be null for the
     // application package, since that's not locked.
     var packageId = graph.lockFile.packages[package.name];
     if (packageId != null &&
-        graph.entrypoint.cache.sources[packageId.source] is CachedSource) {
+        graph.entrypoint.cache.source(packageId.source) is CachedSource) {
       return new Future.value();
     }
 
     var subdirectory = package.path(dir);
     if (!dirExists(subdirectory)) return new Future.value();
 
     // TODO(nweiz): close this watcher when [barback] is closed.
     var watcher = _watcherType.create(subdirectory);
     var subscription = watcher.events.listen((event) {
       // Don't watch files symlinked into these directories.
@@ skipping 164 matching lines @@
   String toString() => "polling";
 }
 
 class _NoneWatcherType implements WatcherType {
   const _NoneWatcherType();
 
   DirectoryWatcher create(String directory) => null;
 
   String toString() => "none";
 }