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And first problem.

According to the spec - es6 classes can not be called as a functions.

Few solutions:

1. Make a breaking change and disalow var map = Map() syntax in favor of var map = new Map().
2. Refactor all es6 classes to Function.prototype style.
3. Keep using declassify (but first - need to fix it for latest rollup).

Hey @umidbekkarimov thanks for digging into this!

#742 is a master task for modernification, it's important for each actual change to be focused and incremental. Could you separate this into separate pull requests?

Done 👍. I'm closing this PR for now.