Events

[keithamus]

This rewrites the events to be a little more type safe, by using custom classes for each event type. This also adds get/set properties which can make it possible to bind these events when used in React.

Refs https://github.com/github/primer/issues/2138

[primer-css]

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[rezrah]

LGTM.. though i'm not super familiar with the source. Seems to do what it says on the tin though, so 👍.