Fix: no-duplicate-case false positives on Object.prototype keys

[mdjermanovic]

What is the purpose of this pull request? (put an "X" next to item)

[X] Bug fix

Tell us about your environment

• ESLint Version: 6.1.0
• Node Version: 10.16.0
• npm Version: 6.9.0

What parser (default, Babel-ESLint, etc.) are you using?

default

Please show your full configuration:

Configuration

module.exports = {
  parserOptions: {
    ecmaVersion: 2015,
  },
};

What did you do? Please include the actual source code causing the issue.

/*eslint no-duplicate-case: "error"*/

switch (foo) {
  case toString:
    break;
}

Demo link

What did you expect to happen?

No warnings.

What actually happened? Please include the actual, raw output from ESLint.

  4:3  error  Duplicate case label  no-duplicate-case

What changes did you make? (Give an overview)

Use Set instead of {}.

Is there anything you'd like reviewers to focus on?

I've added a couple of test cases, only "switch(a) { case toString: break; }" was failing before.

[platinumazure]

LGTM, thanks for contributing!

[kaicataldo]

LGTM. Thanks for contributing!