nassh: lint: update language keyword

Newer closure-compilers have dropped "ECMASCRIPT8", so switch to the
official "ECMASCRIPT_2017" name.

Change-Id: I308f126ae91c053e9b7d4d7d9a4b78cfd058328d
Reviewed-on: https://chromium-review.googlesource.com/1062105
Reviewed-by: Vitaliy Shipitsyn <vsh@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/nassh/bin/lint.sh b/nassh/bin/lint.sh
index 13ae49b..8888722 100755
--- a/nassh/bin/lint.sh
+++ b/nassh/bin/lint.sh
@@ -13,5 +13,5 @@
 exec closure-compiler \
   --checks-only \
   --jscomp_warning=lintChecks \
-  --language_in=ECMASCRIPT8 \
+  --language_in=ECMASCRIPT_2017 \
   "$@"