util should be polyfilled for mocha

[PaperStrike]

Describe the bug
mocha uses util and without this it can't be bundled up. Currently no errors in the test cases as the browser-compatible util is listed as a dep of assert and the test cases in this repo run with it.

To Reproduce

git clone ...
git switch -c test-mocha-util
yarn install
yarn remove assert

create a file named example.js, with the following lines:

it('should work', () => {});

run node ./cli.js example.js, and you will see the error.

[PaperStrike]

I found there is a better way to solve this, other than listing util as a dependency, we can use mocha's official mocha-es2018.js bundle.

[PaperStrike]

The question is mocha.js or mocha-es2018.js, since the latter is only there since 9.0.1 (mochajs/mocha#4657).