Change the flags file so it uses the get function.

plugin/ale/aaflags.vim

@@ -9,45 +9,30 @@ endif
 let g:loaded_ale_flags = 1
 
 " This flag can be set to 0 to disable linting when text is changed.
-if !exists('g:ale_lint_on_text_changed')
-    let g:ale_lint_on_text_changed = 1
-endif
+let g:ale_lint_on_text_changed = get(g:, 'ale_lint_on_text_changed', 1)
 
 " This flag can be set with a number of milliseconds for delaying the
 " execution of a linter when text is changed. The timeout will be set and
 " cleared each time text is changed, so repeated edits won't trigger the
 " jobs for linting until enough time has passed after editing is done.
-if !exists('g:ale_lint_delay')
-    let g:ale_lint_delay = 100
-endif
+let g:ale_lint_delay = get(g:, 'ale_lint_delay', 100)
 
 " This flag can be set to 0 to disable linting when the buffer is entered.
-if !exists('g:ale_lint_on_enter')
-    let g:ale_lint_on_enter = 1
-endif
+let g:ale_lint_on_enter = get(g:, 'ale_lint_on_enter', 1)
 
 " This flag can be set to 0 to disable setting the loclist.
-if !exists('g:ale_set_loclist')
-    let g:ale_set_loclist = 1
-endif
+let g:ale_set_loclist = get(g:, 'ale_set_loclist', 1)
 
 " This flag can be set to 0 to disable setting signs.
-if !exists('g:ale_set_signs')
-    " Enable the flag by default if the 'signs' feature exists.
-    let g:ale_set_signs = has('signs')
-endif
+" This is enabled by default only if the 'signs' feature exists.
+let g:ale_set_signs = get(g:, 'ale_set_signs', has('signs'))
 
 " This flag can be set to 0 to disable echoing when the cursor moves.
-if !exists('g:ale_echo_cursor')
-    let g:ale_echo_cursor = 1
-endif
+let g:ale_echo_cursor = get(g:, 'ale_echo_cursor', 1)
 
 " This flag can be set to 0 to disable warnings for trailing whitespace
-if !exists('g:ale_warn_about_trailing_whitespace')
-    let g:ale_warn_about_trailing_whitespace = 1
-endif
+let g:ale_warn_about_trailing_whitespace =
+\   get(g:, 'ale_warn_about_trailing_whitespace', 1)
 
 " This flag can be set to 1 to keep sign gutter always open
-if !exists('g:ale_sign_column_always')
-    let g:ale_sign_column_always = 0
-endif
+let g:ale_sign_column_always = get(g:, 'ale_sign_column_always', 0)