Adjust spec parser to use exit code 0 when parsing no files
Adjust spec parser to use exit code 0 when parsing no files

Invocations of the spec parser used to exit with an error code (245)
in the case where no files were specified. This was considered to be a
useless type of invocation, and the spec parser would print out a help
message.

However, it is perfectly possible for a Gerrit CL to give rise to
invocations like that, e.g., when the CL deletes some files and does
nothing else. Also, it causes no known issues to report a success exit
code (0) in the case where no files are specified, and hence this CL
changes the spec parser accordingly.

Change-Id: I9bd75839f1a8052b3daa62fa5440451e46fb59e8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/253280
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Commit-Queue: Erik Ernst <251bb01623bfda2cd9151e339a6e783ca432874c@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk