Refactor Source and SourceRegistry.

test/pubspec_test.dart

 // Copyright (c) 2012, 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/src/package.dart';
 import 'package:pub/src/pubspec.dart';
 import 'package:pub/src/source.dart';
 import 'package:pub/src/source/path.dart';
 import 'package:pub/src/source_registry.dart';
+import 'package:pub/src/system_cache.dart';
 import 'package:pub_semver/pub_semver.dart';
 import 'package:test/test.dart';
 
 class MockSource extends Source {
   final String name = "mock";
 
-  Future<List<PackageId>> doGetVersions(PackageRef ref) =>
-      throw new UnsupportedError("Cannot get mock package versions.");
-
-  Future<Pubspec> doDescribe(PackageId id) => throw new UnsupportedError(
-      "Cannot describe mock packages.");
-
-  Future get(PackageId id, String symlink) => throw new UnsupportedError(
-      "Cannot get a mock package.");
-
-  String getDirectory(PackageId id) => throw new UnsupportedError(
-      "Cannot get the directory for mock packages.");
+  LiveSource bind(SystemCache cache) =>
+      throw new UnsupportedError("Cannot download mock packages.");
 
   PackageRef parseRef(String name, description, {String containingPath}) {
     if (description != 'ok') throw new FormatException('Bad');
     return new PackageRef(name, this.name, description);
   }
 
   PackageId parseId(String name, Version version, description) =>
       new PackageId(name, this.name, version, description);
 
   bool descriptionsEqual(description1, description2) =>
@@ skipping 492 matching lines @@
       test("uses the key if the value is null", () {
         var pubspec = new Pubspec.parse('''
 executables:
   command:
 ''', sources);
         expect(pubspec.executables['command'], equals('command'));
       });
     });
   });
 }