Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/barback.dart

 // Copyright (c) 2013, 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.barback;
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
-
-import 'version.dart';
+import 'package:pub_semver/pub_semver.dart';
 
 /// The currently supported versions of packages that this version of pub works
 /// with.
 ///
 /// Pub implicitly constrains these packages to these versions as long as
 /// barback is a dependency.
 ///
 /// Users' transformers are loaded in an isolate that uses the entrypoint
 /// package's dependency versions. However, that isolate also loads code
 /// provided by pub (`asset/dart/transformer_isolate.dart` and associated
@@ skipping 60 matching lines @@
   if (parts.length <= index + 1) {
     throw new FormatException(
         'Invalid URL path "${url.path}". Expected package name '
         'after "packages".');
   }
 
   var package = parts[index + 1];
   var assetPath = p.url.join("lib", p.url.joinAll(parts.skip(index + 2)));
   return new AssetId(package, assetPath);
 }