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

[X] Bug fix

Tell us about your environment

• ESLint Version: 6.3.0
• Node Version: 11.12.0
• npm Version: 6.7.0

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

default

Please show your full configuration:

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

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

There is a hole in the ES6 logic for the no-lone-blocks rule. It is set to accept blocks with declarations, but if such a block appears as a lonely child of another block, it should be an error.

// Not caught by the rule prior to this PR
function foo() {
  {
    const x = 4; 
  }
}

I've seen this more than once in the wild - I suspect it can (somewhat) easily happen when you extract a piece of logic into a function.

What did you expect to happen?

Such blocks should be flagged as errors by the rule.

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

No error.

What changes did you make? (Give an overview)

Added extra condition and tests to cover this case.

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

No.

Is this a breaking change?

No.