Add ability to name isolates for debugging

[Hixie]

In flutter/flutter#19490 @rmacnak-google said "the framework's use of wrapper functions makes it impossible to tell just from the isolate's name". It would be great if we could provide information to Isolate.spawn to make future debugging easier, since we actually have name information at hand when we make that call (we use it in Timeline calls). (I'm not sure where the Isolate's name appears, but presumably in debugging dumps or some such?)

[bkonyi]

Would we consider adding an optional debugName parameter to Isolate.spawn and Isolate.spawnUri a breaking change? I highly doubt anyone has used Isolate as an interface, so I can't see that being an issue, but maybe someone else can think of a situation where this might be breaking?

[natebosch]

I highly doubt anyone has used Isolate as an interface, so I can't see that being an issue, but maybe someone else can think of a situation where this might be breaking?

If we only add it to the static methods Isolate.spawn they aren't part of the interface so I think we should be fine. If we need to add an instance field on Isolate then it might be breaking, but I would be very surprised to see a class implements Isolate.

[natebosch]

Another suggestion from a while ago is to be able to set it from the Service class: #28207