Support for .super from ES6

[cidthecoatrack]

1. Which version of PhantomJS are you using? Tip: run phantomjs --version.
   (I am currently figuring it out, I'm posting this on behalf of Chutzpah, and trying to ascertain the version of PhantomJS it uses)
2. What steps will reproduce the problem?
   1. Run a unit test against a module that calls .super()
   2. super is undefined
   3. When debugging the site or runnign on production, this works fine
3. Which operating system are you using?
4. Did you use binary PhantomJS or did you compile it from source?
5. Please provide any additional information below.
   This in particular prevents us from adding javascript unit tests to our thick client 😢

[cidthecoatrack]

Update: version is 2.1.1

[vitallium]

This feature will be available in upcoming release 2.5.

[ariya]

Let's consolidate it and track it as part of #14506.