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

Start at VMServiceFlutterDriver.connectedTo and trace the dart:io calls that prevent use from a web-based DevTools extension. Determine how the proposed constructor can override those methods, then verify that a DevTools extension can call the driver without importing dart:io; the issue does not name specific files or tests.

Written by the indexing model from the issue text.