Use pub_semver package 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 'package:path/path.dart' as p;
+import 'package:pub_semver/pub_semver.dart';
 import 'package:source_span/source_span.dart';
 import 'package:yaml/yaml.dart';
 
 import 'io.dart';
 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;
 
   /// Creates a new lockfile containing [ids].
   factory LockFile(List<PackageId> ids) {
     var lockFile = new LockFile.empty();
     for (var id in ids) {
@@ skipping 101 matching lines @@
       };
     });
 
     return """
 # Generated by pub
 # See http://pub.dartlang.org/doc/glossary.html#lockfile
 ${yamlToString({'packages': data})}
 """;
   }
 }