devtools extension: Enable use of flutter_driver from a web app

### Use case

I'm writing a devtools extension and I'd like to include some remote driving of the target application, however `flutter_driver` can't be integrated into devtools as it depends on `dart:io`.

It would be useful to be able to call `VMServiceFlutterDriver.connectedTo` from devtools, which it cannot currently do because of calls into `dart:io`.

### Proposal

A constructor that allows overriding of the methods usually delegated to dart:io