Use pub_semver package in pub.

sdk/lib/_internal/pub_generated/lib/src/solver/version_solver.dart

 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';
 Future<SolveResult> resolveVersions(SolveType type, SourceRegistry sources,
     Package root, {LockFile lockFile, List<String> useLatest}) {
   if (lockFile == null) lockFile = new LockFile.empty();
   if (useLatest == null) useLatest = [];
   return log.progress('Resolving dependencies', () {
     return new BacktrackingSolver(
         type,
@@ skipping 93 matching lines @@
     if (error != null) {
       _versionCacheHits++;
       return new Future.error(error.first, error.last);
     }
     _versionCacheMisses++;
     var source = _sources[package.source];
     return source.getVersions(
         package.name,
         package.description).then((versions) {
       versions.sort(
-          _type == SolveType.DOWNGRADE ? Version.antiPrioritize : Version.prioritize);
+          _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) {
       log.solver("Could not get versions for $package:\n$error\n\n$trace");
       _versionErrors[package] = new Pair(error, new Chain.forTrace(trace));
       throw error;
     });
   }
@@ skipping 124 matching lines @@
   }
 }
 class DependencyNotFoundException extends SolveFailure {
   final PackageNotFoundException _innerException;
   String get _message => "${_innerException.message}\nDepended on by";
   DependencyNotFoundException(String package, this._innerException,
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
   String _describeDependency(PackageDep dep) => "";
 }