Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/source.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.source;
 
 import 'dart:async';
 
+import 'package:pub_semver/pub_semver.dart';
+
 import 'package.dart';
 import 'pubspec.dart';
 import 'system_cache.dart';
-import 'version.dart';
 
 /// A source from which to get packages.
 ///
 /// Each source has many packages that it looks up using [PackageId]s. Sources
 /// that inherit this directly (currently just [PathSource]) are *uncached*
 /// sources. They deliver a package directly to the package that depends on it.
 ///
 /// Other sources are *cached* sources. These extend [CachedSource]. When a
 /// package needs a dependency from a cached source, it is first installed in
 /// the [SystemCache] and then acquired from there.
@@ skipping 154 matching lines @@
   /// according to [parseDescription], although it must still be serializable
   /// to JSON and YAML. It must also be equal to [id] according to
   /// [descriptionsEqual].
   ///
   /// By default, this just returns [id].
   Future<PackageId> resolveId(PackageId id) => new Future.value(id);
 
   /// Returns the source's name.
   String toString() => name;
 }