Use pub_semver package in pub.

sdk/lib/_internal/pub_generated/lib/src/lock_file.dart

 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';
 class LockFile {
   Map<String, PackageId> packages;
   factory LockFile(List<PackageId> ids) {
     var lockFile = new LockFile.empty();
     for (var id in ids) {
       if (!id.isRoot) lockFile.packages[id.name] = id;
     }
     return lockFile;
   }
   LockFile._(this.packages);
@@ skipping 68 matching lines @@
         'description': description
       };
     });
     return """
 # Generated by pub
 # See http://pub.dartlang.org/doc/glossary.html#lockfile
 ${yamlToString({'packages' : data})}
 """;
   }
 }