Fix return type warning in pub source.

BUG=
R=ricow@google.com

Review URL: https://codereview.chromium.org//753063003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42063 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/global_packages.dart

@@ -378,7 +378,7 @@ class GlobalPackages {
   void listActivePackages() {
     if (!dirExists(_directory)) return;
 
-    return listDir(_directory).map(_loadPackageId).toList()
+    listDir(_directory).map(_loadPackageId).toList()
         ..sort((id1, id2) => id1.name.compareTo(id2.name))
         ..forEach((id) => log.message(_formatPackage(id)));
   }