Re: [heycam/webidl] Specify open dictionaries. (#180)

@bzbarsky I don't see "is already present" in the spec. Do you mean the "If _typedKey_ is already a key in _result_," wording? That's the same operation as in [`Map.prototype.set`](https://tc39.github.io/ecma262/#sec-map.prototype.set), which I believe we implement efficiently. I do think we want to avoid the possibility of a `record` winding up with a duplicate key: it can only happen with a proxy object input, and spec authors are unlikely to consider it and handle it well.

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