Refactor Source and SourceRegistry.

lib/src/source/unknown.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 'package:pub_semver/pub_semver.dart';
 
 import '../package.dart';
 import '../pubspec.dart';
 import '../source.dart';
+import '../system_cache.dart';
 
 /// A [Null Object] that represents a source not recognized by pub.
 ///
 /// It provides some default behavior so that pub can work with sources it
 /// doesn't recognize.
 ///
 /// [null object]: http://en.wikipedia.org/wiki/Null_Object_pattern
 class UnknownSource extends Source {
   final String name;
 
   UnknownSource(this.name);
 
+  BoundSource bind(SystemCache systemCache) =>
+      new _BoundUnknownSource(this, systemCache);
+
   /// Two unknown sources are the same if their names are the same.
   bool operator==(other) =>
       other is UnknownSource &&
       other.name == name;
 
   int get hashCode => name.hashCode;
 
-  Future<List<PackageId>> doGetVersions(PackageRef ref) =>
-      throw new UnsupportedError(
-          "Cannot get package versions from unknown source '$name'.");
-
-  Future<Pubspec> doDescribe(PackageId id) => throw new UnsupportedError(
-      "Cannot describe a package from unknown source '$name'.");
-
-  Future get(PackageId id, String symlink) => throw new UnsupportedError(
-      "Cannot get an unknown source '$name'.");
-
-  /// Returns the directory where this package can be found locally.
-  String getDirectory(PackageId id) => throw new UnsupportedError(
-      "Cannot find a package from an unknown source '$name'.");
-
   bool descriptionsEqual(description1, description2) =>
       description1 == description2;
 
   PackageRef parseRef(String name, description, {String containingPath}) =>
       new PackageRef(name, this.name, description);
 
   PackageId parseId(String name, Version version, description) =>
       new PackageId(name, this.name, version, description);
 }
+
+class _BoundUnknownSource extends BoundSource {
+  final UnknownSource source;
+
+  final SystemCache systemCache;
+
+  _BoundUnknownSource(this.source, this.systemCache);
+
+  Future<List<PackageId>> doGetVersions(PackageRef ref) =>
+      throw new UnsupportedError(
+          "Cannot get package versions from unknown source '${source.name}'.");
+
+  Future<Pubspec> doDescribe(PackageId id) => throw new UnsupportedError(
+      "Cannot describe a package from unknown source '${source.name}'.");
+
+  Future get(PackageId id, String symlink) => throw new UnsupportedError(
+      "Cannot get an unknown source '${source.name}'.");
+
+  /// Returns the directory where this package can be found locally.
+  String getDirectory(PackageId id) => throw new UnsupportedError(
+      "Cannot find a package from an unknown source '${source.name}'.");
+}