white lists getter and setter acceseors for having stage-2 decorators (#756)

[mbehzad]

• adds get and set kind in addition to method to the list of allowed class members for having a decorator,
• adds tests for this two cases (decorator + set and decorator + get)

Q	A
Bug fix?	yes
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	yes/no
Tests added/pass?	yes
Fixed tickets	#756
License	MIT

For stage 2 decorators, the parseClassBody checks if the decorator is attached to a class method. aka its kind is method. And throws an error if not.
commit af08e92 adds set and get to the class members which are allowed to have a decorator. (extends the check to kind === set || get in addition to method

[mbehzad]

I don't know why the travis build failed. the tests work locally fine. Except the linting which throws error in types.js which was last changed with the usage of prettier in commit 5180ecd (#600) couple of months ago. 🤔

[danez]

Thank you, lgtm

[danez]

Thank you very much for your first pull request.