Use pub_semver package in pub.

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

Index: sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart b/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
index 5991c9bd6824809312d53b7f6859585e1bc0cea3..bc2dfb12e35258c6cd3b6d5f89ea91ac543606f5 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/version_solver.dart
@@ -7,6 +7,7 @@ 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';
@@ -15,7 +16,6 @@ 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';
@@ -226,7 +226,7 @@ class PubspecCache {
     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(