Update: ensure operator-assignment handles exponentiation operators

[not-an-aardvark]

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

[x] Bug fix

Tell us about your environment

• ESLint Version: master
• Node Version: 7.4.0
• npm Version: 4.0.5

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

default

Please show your full configuration:

rules:
  operator-assignment: error
parserOptions:
  ecmaVersion: 7

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

foo = foo ** bar;

What did you expect to happen?

I expected an error to be reported, suggesting that the code use a compound operator.

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

No error was reported.

What changes did you make? (Give an overview)

This updates operator-assignment to identify that the ** operator can be replaced with a compound operator.

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

Nothing in particular

[mention-bot]

@not-an-aardvark, thanks for your PR! By analyzing the history of the files in this pull request, we identified @vitorbal, @btmills and @nzakas to be potential reviewers.

[eslintbot]

LGTM