Make pub strong-mode clean.

lib/src/command/barback.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 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
 
 import '../command.dart';
@@ skipping 93 matching lines @@
     // Make sure all of the source directories exist.
     var missing = sourceDirectories.where(
         (dir) => !dirExists(entrypoint.root.path(dir)));
 
     if (missing.isNotEmpty) {
       dataError(_directorySentence(missing, "does", "do", "not exist"));
     }
 
     // Make sure the directories don't overlap.
     var sources = sourceDirectories.toList();
-    var overlapping = new Set();
+    var overlapping = new Set<String>();
     for (var i = 0; i < sources.length; i++) {
       for (var j = i + 1; j < sources.length; j++) {
         if (path.isWithin(sources[i], sources[j]) ||
             path.isWithin(sources[j], sources[i])) {
           overlapping.add(sources[i]);
           overlapping.add(sources[j]);
         }
       }
     }
 
@@ skipping 62 matching lines @@
     var verb = pluralize(singularVerb, directoryNames.length,
         plural: pluralVerb);
 
     var result = "$directories $names $verb";
     if (suffix != null) result += " $suffix";
     result += ".";
 
     return result;
   }
 }