assert false could produce better error message for diagnose

# Conflicts:
#	lib/4.06.1/unstable/js_compiler.ml
#	lib/4.06.1/unstable/js_refmt_compiler.ml
#	lib/4.06.1/whole_compiler.ml

Author: bobzhang

jscomp/main/bspack_main.ml

@@ -134,7 +134,7 @@ let interface sourcefile =
       ast in
   ast, content
 
-let emit_line_directive = ref false
+let emit_line_directive = ref true
 
 let emit out_chan name =
   if !emit_line_directive then begin