server side api has Cookie, please add to client side api

**Issue by [trinarytree](https://github.com/trinarytree)**
_Originally opened as dart-lang/sdk#19005_

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https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart-io.Cookie
seems to only be available to the server.

that forces other libraries, e.g.
https://docs.angulardart.org/#angular-core.Cookies to implement their own;
or (gasp) the app level code must use the document.cookie api directly.

the angular one isn't nearly as comprehensive as the dart one, e.g.
it doesn't fully support cookie path or expiration dates
(it has really limited support for cookie path).
but really, angular shouldn't have to write such a library,
it should be in dart. you guys already wrote 90% of the code,
you should just share some of it with the client side.
i don't want to have to implement my own crazy DateTime to cookie-date converters.