Re: [whatwg/dom] Define a 'CancelationController' and 'CancelationSignal' interface. (#434)

@annevk there's lots of them - `function isMap(value) { try { Map.prototype.has.call(value); return true; } catch (e) { return false; } }` works fine ([link](https://tc39.github.io/ecma262/#sec-map.prototype.has))

However, I'd encourage you to follow the pattern of `Array.isArray`, since having to catch exceptions is unergonomic and slow.

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