Start adding support for lock files.

utils/pub/pubspec.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');
 
-#import('io.dart');
 #import('package.dart');
 #import('source.dart');
 #import('source_registry.dart');
 #import('utils.dart');
 #import('version.dart');
 #import('yaml/yaml.dart');
 
 /**
  * The parsed and validated contents of a pubspec file.
  */
@@ skipping 81 matching lines @@
         source.validateDescription(description);
 
         dependencies.add(new PackageRef(
             name, source, Version.none, description));
       });
     }
 
     return new Pubspec._(version, dependencies);
   }
 }