Formatter: Fix error-prone warning.

The same fix has been applied upstream in OpenJdk8.

The only thing this fix affects is an exception error message. I am
not writing a CTS test because I don't want to give any sort of false
hope that the exception message is stable.

bug: 27680061
Change-Id: I6281d6a96722684d1ec5fc9bf33ad6c402d125c6
1 file changed