private IndexedSkypeMailFolder createIndexedSkypeMailFolder(FolderIndexMock folderIndex) {
    final FolderMock nonIndexedFolder = new FolderMock();
    folderIndex.addSignatureForId("#42$foo", "previous-signature");

    NonIndexGmailFolderProvider nonIndexedFolderMock =
        new NonIndexGmailFolderProvider() {
          @Override
          public SkypeMailFolder get() {
            return nonIndexedFolder;
          }
        };
    IndexedSkypeMailFolder indexedGmailFolder =
        new IndexedSkypeMailFolder(nonIndexedFolderMock, folderIndex);
    addPreviousMessages(indexedGmailFolder);
    return indexedGmailFolder;
  }