Add dict to function refferencing self

This is no explicit problem in Vim, however Neovim diverged in this
behaviour from Vim and requires the dict attribute to be present before
accessing the self attribute.

See neovim/neovim#5763

autoload/airline/builder.vim

@@ -5,20 +5,20 @@ scriptencoding utf-8
 
 let s:prototype = {}
 
-function! s:prototype.split(...)
+function! s:prototype.split(...) dict
   call add(self._sections, ['|', a:0 ? a:1 : '%='])
 endfunction
 
-function! s:prototype.add_section_spaced(group, contents)
+function! s:prototype.add_section_spaced(group, contents) dict
   let spc = empty(a:contents) ? '' : g:airline_symbols.space
   call self.add_section(a:group, spc.a:contents.spc)
 endfunction
 
-function! s:prototype.add_section(group, contents)
+function! s:prototype.add_section(group, contents) dict
   call add(self._sections, [a:group, a:contents])
 endfunction
 
-function! s:prototype.add_raw(text)
+function! s:prototype.add_raw(text) dict
   call add(self._sections, ['', a:text])
 endfunction
 
@@ -34,7 +34,7 @@ function! s:get_prev_group(sections, i)
   return ''
 endfunction
 
-function! s:prototype.build()
+function! s:prototype.build() dict
   let side = 1
   let line = ''
   let i = 0

autoload/airline/util.vim

@@ -82,7 +82,7 @@ endif
 " available. This way we avoid overwriting v:shell_error, which might
 " potentially disrupt other plugins.
 if has('nvim')
-  function! s:system_job_handler(job_id, data, event)
+  function! s:system_job_handler(job_id, data, event) dict
     if a:event == 'stdout'
       let self.buf .=  join(a:data)
     endif