CP-2239 Support synchronous XHR in the browser

[evanweible-wf]

XMLHttpRequest supports sending requests synchronously. One example of this being useful is sending a request during a window's unload event - an async request won't actually be sent because the window will unload as soon as the current execution stack is finished.

We should support this, although it will be browser-only (throw UnsupportedError if configured for VM).

This will either require a new optional sync parameter or an altogether new API specifically for sending synchronous requests depending on how much of the async code we can avoid in the request implementation.

Most importantly, it is dependent on this bug in the SDK: dart-lang/sdk#24637