Fix #794 - Filter out any preceding lines of Flow output which aren't JSON

ale_linters/javascript/flow.vim

@@ -40,10 +40,26 @@ function! ale_linters#javascript#flow#GetCommand(buffer, version_lines) abort
     \   . ' --json --from ale %s'
 endfunction
 
-function! ale_linters#javascript#flow#Handle(buffer, lines) abort
-    let l:str = join(a:lines, '')
+" Filter lines of flow output until we find the first line where the JSON
+" output starts.
+function! s:GetJSONLines(lines) abort
+    let l:start_index = 0
 
-    if l:str ==# ''
+    for l:line in a:lines
+        if l:line[:0] ==# '{'
+            break
+        endif
+
+        let l:start_index += 1
+    endfor
+
+    return a:lines[l:start_index :]
+endfunction
+
+function! ale_linters#javascript#flow#Handle(buffer, lines) abort
+    let l:str = join(s:GetJSONLines(a:lines), '')
+
+    if empty(l:str)
         return []
     endif
 

test/handler/test_flow_handler.vader

@@ -7,6 +7,22 @@ After:
   unlet! g:actual
   call ale#linter#Reset()
 
+Execute(The flow handler should throw away non-JSON lines):
+  AssertEqual
+  \ [],
+  \ ale_linters#javascript#flow#Handle(bufnr(''), [
+  \   'Already up-to-date.',
+  \   '{"flowVersion":"0.50.0","errors":[],"passed":true}',
+  \ ])
+  AssertEqual
+  \ [],
+  \ ale_linters#javascript#flow#Handle(bufnr(''), [
+  \   'foo',
+  \   'bar',
+  \   'baz',
+  \   '{"flowVersion":"0.50.0","errors":[],"passed":true}',
+  \ ])
+
 Execute(The flow handler should process errors correctly.):
   silent! noautocmd file /home/w0rp/Downloads/graphql-js/src/language/parser.js