Bump @babel/polyfill from 7.2.5 to 7.4.0 in /js

Bumps [@babel/polyfill](https://github.com/babel/babel) from 7.2.5 to 7.4.0.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/compare/v7.2.5...v7.4.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

js/package.json

@@ -23,7 +23,7 @@
   },
   "devDependencies": {
     "@babel/core": "^7.4.0",
-    "@babel/polyfill": "^7.2.5",
+    "@babel/polyfill": "^7.4.0",
     "@babel/preset-env": "^7.4.2",
     "babel-loader": "^8.0.5",
     "css-loader": "^2.1.1",