Make VersionSolver source- and description-aware.

utils/tests/pub/lock_file_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.
 
 #library('lock_file_test');
 
 #import('../../../lib/unittest/unittest.dart');
 #import('../../pub/lock_file.dart');
 #import('../../pub/package.dart');
 #import('../../pub/source.dart');
 #import('../../pub/source_registry.dart');
 #import('../../pub/utils.dart');
 #import('../../pub/version.dart');
 
 class MockSource extends Source {
   final String name = 'mock';
   final bool shouldCache = false;
 
   void validateDescription(String description) => description.endsWith(' desc');
 
-  String packageName(PackageId id) {
+  String packageName(String description) {
     // Strip off ' desc'.
-    return id.description.substring(0, id.description.length - 5);
+    return description.substring(0, description.length - 5);
   }
 }
 
 main() {
   var sources = new SourceRegistry();
   var mockSource = new MockSource();
   sources.register(mockSource);
 
   group('LockFile', () {
     group('parse()', () {
@@ skipping 128 matching lines @@
     description: foo desc
 ''', sources);
       });
     });
   });
 }
 
 throwsBadFormat(function) {
   expectThrow(function, (e) => e is FormatException);
 }