This flag is picked up by webpack v4 and used for more agressive optimizations.
Our code is already side-effect free, because that's what we needed for build-optimizer to work.
PR Close#22785
* Remove now unnecessary portions of build.
* Add a compilePackageES5 method to build ES5 from sources
* Rework all package.json and rollup config files to new format
* Remove "extends" from tsconfig-build.json files and fixup compilation roots
PR Close#18541
Igor Minar
Jason Aden