Add dedicated tabline builder

autoload/airline/extensions/tabline/builder.vim

@@ -0,0 +1,12 @@
+" MIT License. Copyright (c) 2013-2018 Bailey Ling et al.
+" vim: et ts=2 sts=2 sw=2
+
+scriptencoding utf-8
+
+let s:prototype = {}
+
+function! airline#extensions#tabline#builder#new(context)
+  let builder = airline#builder#new(a:context)
+  call extend(builder, s:prototype, 'force')
+  return builder
+endfunction