Refactor Source and SourceRegistry.

lib/src/source/cached.dart

Index: lib/src/source/cached.dart
diff --git a/lib/src/source/cached.dart b/lib/src/source/cached.dart
index e492f54c36af40207284c9afd8a14734c0c66b9d..8a40bc3bcba4864c4e238afbacb75dc4250740e5 100644
--- a/lib/src/source/cached.dart
+++ b/lib/src/source/cached.dart
@@ -12,17 +12,18 @@ import '../pubspec.dart';
 import '../source.dart';
 import '../utils.dart';
 
-/// Base class for a [Source] that installs packages into pub's [SystemCache].
+/// Base class for a [BoundSource] that installs packages into pub's
+/// [SystemCache].
 ///
 /// A source should be cached if it requires network access to retrieve
 /// packages or the package needs to be "frozen" at the point in time that it's
 /// installed. (For example, Git packages are cached because installing from
 /// the same repo over time may yield different commits.)
-abstract class CachedSource extends Source {
+abstract class CachedSource extends BoundSource {
   /// The root directory of this source's cache within the system cache.
   ///
   /// This shouldn't be overridden by subclasses.
-  String get systemCacheRoot => path.join(systemCache.rootDir, name);
+  String get systemCacheRoot => path.join(systemCache.rootDir, source.name);
 
   /// If [id] is already in the system cache, just loads it from there.
   ///