Summary:
This diff adds support for enforcing nullability in the arguments for exported methods.
We previously supported use of the nullable/nonnull attributes on method arguments, but didn't do anything to ensure that they were respected.
Now, if an argument is marked as nonnull, and a null value is sent for that argument, it will display a redbox.
In future, nonnull will be assumed by default, but for now we assume that un-annotated arguments can be null (to avoid breaking existing code).
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