The async keyword should not be followed by a newline

Fixes #1514

Author: vouillon

compiler/lib/js_output.ml

@@ -444,7 +444,7 @@ struct
         (match k with
         | { async = true; generator = false } ->
             PP.string f "async";
-            PP.space f
+            PP.non_breaking_space f
         | { async = false; generator = false } -> ()
         | { async = true | false; generator = true } -> assert false);
         PP.break f;