feat(closure): fix #727, add zone_externs.js for closure compiler

Author: JiaLiPassion

Diff for: .travis.yml

@@ -25,6 +25,7 @@ script:
   - node_modules/.bin/gulp lint
   - node_modules/.bin/gulp format:enforce
   - node_modules/.bin/gulp build
+  - scripts/closure/closure_compiler.sh
   - node_modules/.bin/gulp promisetest
   - node_modules/.bin/karma start karma-dist-sauce-jasmine.conf.js --single-run
   - node_modules/.bin/karma start karma-build-sauce-mocha.conf.js --single-run