restore removed methods which still used

[outsideris]

Description of the Change

In ec8901a , utils.files is removed, but it still used in https://github.com/mochajs/mocha/blob/master/bin/_mocha#L532 . So, running mocha with watch mode raised a error with v5.0.2. See #3265 . We should re-release new version.

mocha --watch

/Users/outsider/temp/mocha-test/node_modules/mocha/bin/_mocha:532
  const watchFiles = utils.files(cwd, [ 'js' ].concat(program.watchExtensions));
                           ^

TypeError: utils.files is not a function
    at Object.<anonymous> (/Users/outsider/temp/mocha-test/node_modules/mocha/bin/_mocha:532:28)
    at Module._compile (module.js:643:30)
    at Object.Module._extensions..js (module.js:654:10)
    at Module.load (module.js:556:32)
    at tryModuleLoad (module.js:499:12)
    at Function.Module._load (module.js:491:3)
    at Function.Module.runMain (module.js:684:10)
    at startup (bootstrap_node.js:187:16)
    at bootstrap_node.js:608:3

Alternate Designs

N/A

Why should this be in core?

Users who use watch mode encounter error.

Benefits

Not break mocha in watch mode.

Possible Drawbacks

N/A

Applicable issues

Close #3265

[coveralls]

Coverage increased (+0.06%) to 90.077% when pulling c7cab5e on outsideris:issue-3265 into aaaa5ab on mochajs:master.

[boneskull]

I'll release this tomorrow