[webidl] Allow unions of dictionaries with a common base dictionary, in return values (#57)

>From https://github.com/w3c/webrtc-pc/issues/275#issuecomment-135623998

tl;dr: This avoids the temptation to write "snowball" dictionaries for the purpose of return values.

Member name collisions not from inheritance would be a compiler error.

Additional potential (mis?)use-case https://github.com/w3c/mediacapture-main/pull/211#discussion_r38549547


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Reply to this email directly or view it on GitHub:
https://github.com/heycam/webidl/issues/57