If a polyfills file is original, then replace completely with latest version of the file. This removes the need to deeply analyze the file as well as ensuring all comments are up to date. Also skips analysis if there is no mention of `core-js` of any form in the file.
The header re-addition check is also reduced in scope to account for varying versions of the header content.
We don't know yet if we are going to separate zone.js in a separate file. Hence, we should remove this for the time being and include it only when we remove zone.js from the main polyfills files
1. Remove imports of es6 polyfills introduced by the CLI.
2. Refactor the migrations for version 8 by moving the codelyzer and
polyfill transforms into different files.
The PR drops all `core-js/es6` polyfills that we've introduced with the
CLI, except the commented ones. We do not remove commented imports,
since they are not part of the internal es6 polyfills.
The migration automatically drops the
associated comments with the removed imports since they are part of the node - under its
`jsDoc` property.
