Currently, when using the performance runtime type check feature, you can yield invalid iterables from a DoFn and not have an error thrown. This is incorrect behavior.
We should throw a TypeCheckError if the iterable type is any of the following: dict, str, unicode, or bytes.
Previously, we tried accomplishing this using an isinstance check in process_outputs of common.py, however this harms performance significantly. We should search for a cheaper way to verify that the output type is valid. One possible solution is to only do the isinstance check if the flag is on.