chore(package): update Babel to RC1

[layershifter]

• update Babel to RC1
• update babel-plugin-istanbul
• use @babel/polyfill instead of outdated babel-polyfill

[codecov-io]

Codecov Report

Merging #3086 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3086   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files         163      163           
  Lines        2739     2739           
=======================================
  Hits         2737     2737           
  Misses          2        2

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