[WIP] Fix: static getters / setters can have incorrect this binding

[matthewjh]

Please see #4327.

There's a modification to the __extends helper that fixes this issue, however I'm unsure how to write tests for this.

I've tried to add a compiler test case to cover this, but it's hard to see how I can use it to check that this's runtime type is correct.

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[mhegazy]

Please see #4327 for more discussion on this topic.