Allow new.target in class properties

[existentialism]

Q	A
Bug fix?	yes
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	yes
Tests added/pass?	yes
Fixed tickets	Fixes #758
License	MIT

[nicolo-ribaudo]

Is new.target allowed in arrow parameters? e.g. class A { x = (p = new.target) => {} }

Also, I think class A { x = (new.target) } should throw.

[jridgewell]

/cc @littledan, what's the expected behavior here?

[existentialism]

Related: babel/babel#6243

[littledan]

new.target would always be undefined in any of these cases, just like in a method.

[nicolo-ribaudo]

Approved too early 😅
Can you add a test for the new.target can only be used in functions or class properties error message, so we get full coverage for this PR?

[littledan]

Would it make sense to add a test that verifies that, when you execute the code, you actually get undefined for new.target in these cases?

[nicolo-ribaudo]

I think we should add them, but they must be in the Babel repository (after this PR is merged)

[nicolo-ribaudo]

LGTM

[hzoo]

This will be better when babylon is merged into the monorepo