It should not be required that elements are JSON serializable

When using the Shady DOM polyfill https://github.com/webcomponents/shadydom you will hit TypeError: cyclic object value when trying to return a list of elements from an execute script call (see https://github.com/webcomponents/shadydom/issues/217).

It seems that webdrivers internally use JSON.stringify on the list of elements to return, which in turn runs JSON.stringify on the elements inside the list. While this works fine for the standard case, the Shady DOM polyfill adds a __shady property to the element and at least when the element contains children, the __shady property contains cycles and cannot thus be coverted to JSON. Now this implementation detail of __shady has seemingly absolutely nothing to do with returning an element from a webdriver but because of JSON.stringify, it won't work.

Returning a single element works fine as JSON.stringify does not seem to be used in that case.

Start by tracing how an execute script call returns a list of elements and where JSON.stringify is applied to the returned values. Reproduce the Shady DOM case described in issue 217, comparing a list with a single element. Done means cyclic __shady data no longer prevents returning element lists while single-element returns continue to work.