Use a lockfile to persist Pub's installed version constellation.

utils/tests/pub/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.
 
 #library('pubspec_test');
 
 #import('../../../lib/unittest/unittest.dart');
 #import('../../pub/pubspec.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(description) {
+  void validateDescription(description, [bool fromLockFile=false]) {
     if (description != 'ok') throw new FormatException('Bad');
   }
   String packageName(description) => 'foo';
 }
 
 main() {
   group('Pubspec', () {
     group('parse()', () {
       test("allows a version constraint for dependencies", () {
         var sources = new SourceRegistry();
@@ skipping 39 matching lines @@
         expect(pubspec.version, equals(Version.none));
         expect(pubspec.dependencies, isEmpty);
       });
     });
   });
 }
 
 throwsBadFormat(function) {
   expectThrow(function, (e) => e is FormatException);
 }