Use pub_semver package in pub.

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

 library pub.package;
 import 'dart:io';
+import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
-import 'package:barback/barback.dart';
+import 'package:pub_semver/pub_semver.dart';
 import 'barback/transformer_id.dart';
 import 'io.dart';
 import 'git.dart' as git;
 import 'pubspec.dart';
 import 'source_registry.dart';
 import 'utils.dart';
-import 'version.dart';
 final _README_REGEXP = new RegExp(r"^README($|\.)", caseSensitive: false);
 class Package {
   static int orderByNameAndVersion(Package a, Package b) {
     var name = a.name.compareTo(b.name);
     if (name != 0) return name;
     return a.version.compareTo(b.version);
   }
   final String dir;
   String get name {
     if (pubspec.name != null) return pubspec.name;
@@ skipping 164 matching lines @@
     return "$name $constraint from $source ($description)";
   }
   int get hashCode => name.hashCode ^ source.hashCode;
   bool operator ==(other) {
     return other is PackageDep &&
         other.name == name &&
         other.source == source &&
         other.constraint == constraint;
   }
 }