Fix membership infering for methods

[NumminorihSF]

This PR resolves #1121.
Improve inferMembership to allow to use next code:

/**
 * @memberof BigFeature
 */
class MyClass {
  /** method */
  method() {}
}

Before this improvement, we have method inside
global.MyClass instead of global.BigFeature.MyClass

[coveralls]

Coverage decreased (-0.2%) to 85.612% when pulling 5a504a2 on NumminorihSF:fix/infer-membership-for-es6-classes into fb37789 on documentationjs:master.

[coveralls]

Coverage increased (+0.1%) to 86.004% when pulling 3059f0b on NumminorihSF:fix/infer-membership-for-es6-classes into fb37789 on documentationjs:master.

[tmcw]

Looks good, mind implementing a testcase to make sure that this doesn't regress?

[NumminorihSF]

I'll try to implement a testcase it a bit later.

[NumminorihSF]

@tmcw done

[tmcw]

👍 Thanks! Published in 8.1.2