Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/package.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.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);
 
 /// A named, versioned, unit of code and resource reuse.
 class Package {
   /// Compares [a] and [b] orders them by name then version number.
   ///
   /// This is normally used as a [Comparator] to pass to sort. This does not
   /// take a package's description or root directory into account, so multiple
   /// distinct packages may order the same.
@@ skipping 351 matching lines @@
 
   bool operator ==(other) {
     // TODO(rnystrom): We're assuming here that we don't need to delve into the
     // description.
     return other is PackageDep &&
            other.name == name &&
            other.source == source &&
            other.constraint == constraint;
   }
 }