function toArrayForTest() {
	      return _toArrayForTest.apply(this, arguments);
	    }
Collect all elements of this dataset into an array with prefetching 100
elements. This is useful for testing, because the prefetch changes the
order in which the Promises are resolved along the processing pipeline.
This may help expose bugs where results are dependent on the order of
Promise resolution rather than on the logical order of the stream (i.e.,
due to hidden mutable state).

@returns A Promise for an array of stream elements, which will resolve
  when the stream is exhausted.