Merge pull request #4545 from Polymer/lint-closure

add gulp task to lint for closure warnings

Author: dfreedm

.travis.yml

@@ -19,6 +19,7 @@ before_script:
 - npm install -g bower gulp-cli@1
 - bower install
 - gulp lint
+- gulp lint-closure
 script:
 - xvfb-run wct
 - if [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then wct -s 'windows 10/microsoftedge@14' -s 'windows 8.1/internet explorer@11' -s 'os x 10.11/safari@9' -s 'macos 10.12/safari@10' -s 'Linux/chrome@41'; fi