Make pub strong-mode clean.

lib/src/command.dart

 // Copyright (c) 2013, 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.
 
 import 'package:args/args.dart';
 import 'package:args/command_runner.dart';
 
 import 'entrypoint.dart';
 import 'log.dart' as log;
 import 'global_packages.dart';
@@ skipping 66 matching lines @@
   }
 
   /// Parses a user-supplied integer [intString] named [name].
   ///
   /// If the parsing fails, prints a usage message and exits.
   int parseInt(String intString, String name) {
     try {
       return int.parse(intString);
     } on FormatException catch (_) {
       usageException('Could not parse $name "$intString".');
+      return null;
     }
   }
 }