pass context into the builder.

autoload/airline/builder.vim

@@ -24,16 +24,15 @@ function! s:prototype.add_raw(text)
 endfunction
 
 function! s:prototype.build()
-  if !self._active
+  if !self._context.active
     let self._line = substitute(self._line, '%#.\{-}\ze#', '\0_inactive', 'g')
   endif
   return self._line
 endfunction
 
-function! airline#builder#new(active, highlighter)
+function! airline#builder#new(context, highlighter)
   let builder = copy(s:prototype)
-  let builder._sections = []
-  let builder._active = a:active
+  let builder._context = a:context
   let builder._highlighter = a:highlighter
   let builder._side = 1
   let builder._curgroup = ''