Stubbing class field-functions in ES7

In my test suite, how can I stub a class' property, which is a function*? With normal methods it's easy using Object.getOwnPropertyNames(component.prototype) and monkey patching each found method, but after a long time of struggle I haven't found any way to extract the functions created by assigning to class' fields.

My testing stack consists of Jest with Jasmine2 and babel.

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(*) The background of this question is unit testing React components written in es7-like classes with arrow functions assigned to class' properties for the purpose of auto binding.