Codegen: throw type error when encountering an unknown argument
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FIX#24130
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Support default toJSON in WebIDL
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Ported related lines from gecko.
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In the `fill` method, it will check if the exception code is empty
string or has newline character in the end of string or not. However, we
didn't do any change to exceptionCode when type is Promise. Thus, we add
the newline character to make it pass the checking in `fill` method.
See more detail from the log of IRC: https://mozilla.logbot.info/servo/20180501#c14692647
