kzc

~~Could you please close #1472 if this PR replaces it?~~ To consider this PR we would need `bin/uglifyjs` and `minify()` tests in `test/mocha/`, as well as tests in `test/compress/`.

Could you please remove the [Harmony] prefix in the title if you intend this to be merged to the branch master?

Also would need documentation in README.md

Could you please change the `bin/uglifyjs` flag name to `mangle-props-prefix`?

@dabretin I apologize for not raising this issue earlier - I'm afraid I don't understand the use case of this PR: https://github.com/mishoo/UglifyJS2/pull/1472#issuecomment-278281444 Why would anyone want to add a prefix...

Need a command line test in `test/mocha/` to test the new `bin/uglifyjs` flag. See `test/mocha/cli.js` for an example.

This PR will not be approved without a command line test for `bin/uglifyjs`. example: https://github.com/mishoo/UglifyJS2/blob/1eaa211e0932105439d98d4f03a981f157f0a77c/test/mocha/cli.js#L93-L102

This PR appears to be low risk and now has sufficient tests and documentation. Please squash into a single commit and assign an appropriate commit comment. Thanks.

If this PR gets in I think it could very well be the last new mangle option. They are getting harder to support and test with each new addition and...

I initially thought using an uninitialized variable instead of `void 0` would incur a runtime symbol lookup cost if used in a hot loop, but I don't see evidence of...