Add a client that uses package:http

[parlough]

Then in a future release, the other clients can be deprecated in favor of this one.

[natebosch]

The main concern at an initial glance is this will add http to the set of packages that are pinned by flutter. @devoncarew I think we should consider whether we are ok with this before getting in to a technical review.

@parlough can you share your motivation for this change?

[parlough]

@parlough can you share your motivation for this change?

The motivation was to no longer require maintaining and choosing between two different clients and to follow the best practice we recommend of using package:http rather than relying on platform specifics.

I didn't realize Flutter pinned this package, but it seems it's only a dev dependency for flutter proper. It is a normal dependency for flutter_tools, but that seems to pin package:http already anyway.

It's also a dependency for flutter_driver, but I'm not sure if that would be a problem.

[natebosch]

It's also a dependency for flutter_driver, but I'm not sure if that would be a problem.

It's still a problem for fllutter_driver. The scope of the problem is smaller than if it was a dep of flutter or flutter_test though. I'll see if I can get another opinion on how much of a problem it is.