Based on a surface reading of the specification, and the existence of JS files in js/src/builtin, I decided to take a first pass at what this might look like, with a dependency on XPConnect's Components. I'm not claiming this is anywhere near correct, nor that it should be implemented in JavaScript. Think of this as a starting point for bootstrapping an implementation, if Mozilla developers consider that desirable.
Bug 1367476 Comment 9 Edit History
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Based on a surface reading of the specification, and the existence of JS files in js/src/builtin, I decided to take a first pass at what this might look like, with a dependency on XPConnect's Components. I'm not claiming this is anywhere near correct, nor that it should be implemented in JavaScript. Think of this as a starting point for bootstrapping an implementation, if Mozilla developers consider that desirable. Note this is completely untested!!