new flymake: cosmetic problem on terminal emacs

[Yuta Yamada]

In terminal Emacs, I found not highlight state of flymake even there is a warning.

It can be visible if you set underline to the flymake's face.

How to reproduce:

1. cd emacs repo to ./lisp/progmode/

2. open js.el: $ emacs -Q -nw

4. M-x flymake-mode

3. goto-line 240 or 568 at js.el

   (there is warning around here, basically putting a space before end of the line

    cause similar warning in docstring)

5. you can check it by flymake-goto-next-error, but it isn't visible.

Not sure this is related, flycheck highlight previous word with the same error.

(checked on Emacs-26 branch and pulled 75bb482. This time I used bootstrap option)

Thanks,

Yuta Yamada