Fix WebUI build

09:23:48 init:
09:23:49      [exec] 
09:23:49      [exec] > nuxeo-web-ui@2.4.29 lint /opt/jenkins/workspace/10.10/addons_nuxeo-web-ui-10.10
09:23:49      [exec] > gulp lint && polymer lint -i elements/*/*.html
09:23:49      [exec] 
09:23:50      [exec] [07:23:50] Using gulpfile ~/workspace/10.10/addons_nuxeo-web-ui-10.10/gulpfile.js
09:23:50      [exec] [07:23:50] Starting 'lint'...
09:23:54      [exec] npm ERR! code ELIFECYCLE
09:23:54      [exec] npm ERR! errno 1
09:23:54      [exec] npm ERR! nuxeo-web-ui@2.4.29 lint: `gulp lint && polymer lint -i elements/*/*.html`
09:23:54      [exec] npm ERR! Exit status 1
09:23:54      [exec] npm ERR! 
09:23:54      [exec] npm ERR! Failed at the nuxeo-web-ui@2.4.29 lint script.
09:23:54      [exec] npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
09:23:54      [exec] 
09:23:54      [exec] npm ERR! A complete log of this run can be found in:
09:23:54      [exec] npm ERR!     /opt/jenkins/.npm/_logs/2020-07-24T07_23_54_032Z-debug.log
09:23:54      [exec] [07:23:54] 
09:23:54      [exec] /opt/jenkins/workspace/10.10/addons_nuxeo-web-ui-10.10/elements/nuxeo-app.html
09:23:54      [exec]   618:11  error  Parsing error: The keyword 'const' is reserved
09:23:54      [exec] 
09:23:54      [exec] ✖ 1 problem (1 error, 0 warnings)
09:23:54      [exec] 
09:23:54      [exec] [07:23:54] 'lint' errored after 3.08 s
09:23:54      [exec] [07:23:54] ESLintError in plugin 'gulp-eslint'
09:23:54      [exec] Message:
09:23:54      [exec]     Failed with 1 error