Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/solver/version_solver.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.solver.version_solver;
 
 import 'dart:async';
 import "dart:convert";
 
+import 'package:pub_semver/pub_semver.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import '../exceptions.dart';
 import '../lock_file.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../pubspec.dart';
 import '../source_registry.dart';
-import '../version.dart';
 import '../utils.dart';
 import 'backtracking_solver.dart';
 import 'solve_report.dart';
 
 /// Attempts to select the best concrete versions for all of the transitive
 /// dependencies of [root] taking into account all of the [VersionConstraint]s
 /// that those dependencies place on each other and the requirements imposed by
 /// [lockFile].
 ///
 /// If [useLatest] is given, then only the latest versions of the referenced
@@ skipping 190 matching lines @@
       _versionCacheHits++;
       return new Future.error(error.first, error.last);
     }
 
     _versionCacheMisses++;
 
     var source = _sources[package.source];
     return source.getVersions(package.name, package.description)
         .then((versions) {
       // Sort by priority so we try preferred versions first.
-      versions.sort(_type == SolveType.DOWNGRADE ? Version.antiPrioritize :
+      versions.sort(_type == SolveType.DOWNGRADE ? Version.antiprioritize :
           Version.prioritize);
 
       var ids = versions.reversed.map(
           (version) => package.atVersion(version)).toList();
       _versions[package] = ids;
       return ids;
     }).catchError((error, trace) {
       // If an error occurs, cache that too. We only want to do one request
       // for any given package, successful or not.
       log.solver("Could not get versions for $package:\n$error\n\n$trace");
@@ skipping 252 matching lines @@
 
   DependencyNotFoundException(String package, this._innerException,
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   /// The failure isn't because of the version of description of the package,
   /// it's the package itself that can't be found, so just show the name and no
   /// descriptive details.
   String _describeDependency(PackageDep dep) => "";
 }