Use pub_semver package in pub.

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

 // Copyright (c) 2014, 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.global_packages;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:barback/barback.dart';
+import 'package:pub_semver/pub_semver.dart';
 
 import 'barback/asset_environment.dart';
 import 'entrypoint.dart';
 import 'executable.dart' as exe;
 import 'io.dart';
 import 'lock_file.dart';
 import 'log.dart' as log;
 import 'package.dart';
 import 'pubspec.dart';
 import 'sdk.dart' as sdk;
 import 'solver/version_solver.dart';
 import 'source/cached.dart';
 import 'source/git.dart';
 import 'source/path.dart';
 import 'system_cache.dart';
 import 'utils.dart';
-import 'version.dart';
 
 /// Matches the package name that a binstub was created for inside the contents
 /// of the shell script.
 final _binStubPackagePattern = new RegExp(r"Package: ([a-zA-Z0-9_-]+)");
 
 /// Maintains the set of packages that have been globally activated.
 ///
 /// These have been hand-chosen by the user to make their executables in bin/
 /// available to the entire system. This lets them access them even when the
 /// current working directory is not inside another entrypoint package.
@@ skipping 610 matching lines @@
           "${log.yellow('Warning:')} Pub installs executables into "
               "${log.bold(binDir)}, which is not on your path.\n"
           "You can fix that by adding this to your shell's config file "
               "(.bashrc, .bash_profile, etc.):\n"
           "\n"
           "  ${log.bold('export PATH="\$PATH":"$binDir"')}\n"
           "\n");
     }
   }
 }