A Nov 2018 change in Airline broke g:base16_airline=1 mode

[chkno]

vim-airline commit 14691bb00e1ce3f8118ec664fc3308ee37a66a55 "themes: load them explicitly rather than implictly" on 2018 Nov 13 broke Soares/base16.nvim 's airline integration. After this commit, attempts to use g:base16_airline=1 yield this error message:

airline: The specified theme "summerfruit_dark" cannot be found.

vim-airline commit ffac12cbbe68577c4431ecabbc9bc142138b01a9 "theme: do not show not-found warning" on 2018 Nov 14 broke it a bit further, changing the error output to:

airline: The specified theme "summerfruit_dark" cannot be found.
Error detected while processing function airline#switch_theme:
line   11:
E605: Exception not caught: not-found
Error detected while processing 
/home/username/.config/nvim/plugged/base16.nvim/colors/summerfruit.vim:
line  418:
E171: Missing :endif

A minimal .config/nvim/init.vim configuration that reproduces this problem:

set termguicolors

call plug#begin()
Plug 'vim-airline/vim-airline'
Plug 'Soares/base16.nvim'
call plug#end()

let g:base16_airline = 1
set background=dark
colorscheme summerfruit

A git-bisect session indicating these airline commits.

[chkno]

As a workaround, one can pull in an old version of Airline from just before this change like this:

Plug 'vim-airline/vim-airline', { 'commit': 'b20e181bc8235d48ca02093322f7822fd3719c17' }

[chkno]

All the standard base16 themes were added to airline-themes in Feb 2015.