Refactor Source and SourceRegistry.

lib/src/lock_file.dart

Index: lib/src/lock_file.dart
diff --git a/lib/src/lock_file.dart b/lib/src/lock_file.dart
index 7810bad1ddfcb60f447aa81339ad2e337092df4b..c5ac4b43885fc1b92bd3c6ffd6db132f87baaee6 100644
--- a/lib/src/lock_file.dart
+++ b/lib/src/lock_file.dart
@@ -13,6 +13,7 @@ import 'package:yaml/yaml.dart';
 import 'io.dart';
 import 'package.dart';
 import 'source_registry.dart';
+import 'system_cache.dart';
 import 'utils.dart';
 
 /// A parsed and validated `pubspec.lock` file.
@@ -175,12 +176,12 @@ class LockFile {
   ///
   /// If [entrypoint] is passed, a relative entry is added for its "lib/"
   /// directory.
-  String packagesFile([String entrypoint]) {
+  String packagesFile(SystemCache cache, [String entrypoint]) {
     var header = "Generated by pub on ${new DateTime.now()}.";
 
     var map = new Map.fromIterable(ordered(packages.keys), value: (name) {
       var id = packages[name];
-      var source = _sources[id.source];
+      var source = cache.source(id.source);
       return p.toUri(p.join(source.getDirectory(id), "lib"));
     });