Remove unused imports in pub.

sdk/lib/_internal/pub/lib/src/lock_file.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 pub.lock_file;
 
-import 'dart:collection';
-
 import 'package:yaml/yaml.dart';
 
 import 'io.dart';
-import 'log.dart' as log;
 import 'package.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 import 'version.dart';
 
 /// A parsed and validated `pubspec.lock` file.
 class LockFile {
   /// The packages this lockfile pins.
   Map<String, PackageId> packages;
 
@@ skipping 75 matching lines @@
       };
     });
 
     return """
 # Generated by pub
 # See http://pub.dartlang.org/doc/glossary.html#lockfile
 ${yamlToString({'packages': data})}
 """;
   }
 }