Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/command/global_activate.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.command.global_activate;
 
 import 'dart:async';
 
+import 'package:pub_semver/pub_semver.dart';
+
 import '../command.dart';
 import '../utils.dart';
-import '../version.dart';
 
 /// Handles the `global activate` pub command.
 class GlobalActivateCommand extends PubCommand {
   String get description => "Make a package's executables globally available.";
   String get usage => "pub global activate <package...>";
   bool get takesArguments => true;
 
   GlobalActivateCommand() {
     commandParser.addOption("source",
         abbr: "s",
@@ skipping 71 matching lines @@
       case "path":
         var path = readArg("No package to activate given.");
         validateNoExtraArgs();
         return globals.activatePath(path, executables,
             overwriteBinStubs: overwrite);
     }
 
     throw "unreachable";
   }
 }