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As pointed out in this thread:
Once we have the capability to annotate methods as returning 'not null', we have a win if standard runtime methods are so annotated as appropiate.
Confirmed, this one is harder than the new operator.
The way we can mark the function/function type is in build_method_invocation,
we have to change how BUILD_APPEND calls build_method_invocation and add a new argument or something like that (we build the real call expression later on).