Build: Only transpile JS which is not supported based on chrome version

[gyandeeps]

Did you search for similar issues before submitting this one?
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Describe the issue you encountered:

Currently, we transpile all the JavaScript to ES5 code. But I think we can be smart by using https://github.com/babel/babel-preset-env to only tranpile code which is not supported by a particular version of chrome (I guess we can say 54 now).

Expected behavior:

This will help us to run most of the code we write as it is. Chrome as better perfs on certain ES6 features i guess.

• Platform (Win7, 8, 10? macOS? Linux distro?):
  all
• Brave Version (revision SHA):
  master