Support "pub downgrade".

sdk/lib/_internal/pub/lib/src/command/global_deactivate.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_deactivate;
 
 import 'dart:async';
 
 import '../command.dart';
 import '../utils.dart';
-import '../version.dart';
 
 /// Handles the `global deactivate` pub command.
 class GlobalDeactivateCommand extends PubCommand {
   String get description => "Remove a previously activated package.";
   String get usage => "pub global deactivate <package>";
   bool get requiresEntrypoint => false;
   bool get takesArguments => true;
 
   Future onRun() {
     // Make sure there is a package.
     if (commandOptions.rest.isEmpty) {
       usageError("No package to deactivate given.");
     }
 
     // Don't allow extra arguments.
     if (commandOptions.rest.length > 1) {
       var unexpected = commandOptions.rest.skip(1).map((arg) => '"$arg"');
       var arguments = pluralize("argument", unexpected.length);
       usageError("Unexpected $arguments ${toSentence(unexpected)}.");
     }
 
     globals.deactivate(commandOptions.rest.first);
+    return null;
   }
 }