Merge branch 'patch-8' into 'master'

Add ES array methods as cause of Phantom.js errors.

## What does this MR do?

Adds another example of something that causes a common error in JavaScript testing to the frontend dev docs.

See merge request !7102

doc/development/frontend.md

@@ -238,13 +238,18 @@ For our currently-supported browsers, see our [requirements][requirements].
 [scss-style-guide]: scss_styleguide.md
 [requirements]: ../install/requirements.md#supported-web-browsers
-## Common Errors
-### Rspec (Capybara/Poltergeist) chokes on general JavaScript errors
+## Gotchas
+### Phantom.JS (used by Teaspoon & Rspec) chokes, returning vague JavaScript errors
 If you see very generic JavaScript errors (e.g. `jQuery is undefined`) being thrown in tests, but
 can't reproduce them manually, you may have included `ES6`-style JavaScript in files that don't
 have the `.js.es6` file extension. Either use ES5-friendly JavaScript or rename the file you're
-working in (`git mv <file>.js> <file.js.es6>`).
+working in (`git mv <file.js> <file.js.es6>`). 
+
+Similar errors will be thrown if you're using 
+any of the [array methods introduced in ES6](http://www.2ality.com/2014/05/es6-array-methods.html)
+whether or not you've updated the file extension.
+