Support running and debugging flutter applications

This is a general issue for supporting using this plugin for Flutter developments (flutter.io). In broad strokes, support might look like:

• a preference for where the flutter sdk lives
• starting a copy of the flutter daemon if the user is working on a flutter project
• showing the connected mobile devices in the status line (we get this info from the flutter daemon)
• creating a new flutter launch configuration type
• talking over the daemon protocol to start and stop flutter apps on devices
• on app start, when a service protocol port becomes available, start a debug connection with the app

TODO

• fix debugging/stepping
• allow user to select device if multiple devices connected
• implement hot reload
• fix debug restarting (fixed in Code https://github.com/Microsoft/vscode/issues/28175)
• tidy up debug argments (move to class, make strongly typed)
• remove all flutter-related things from Dart debugger and just add in the subclass
• remove support for port and always use wsUri

[DanTup]

@jhandguy I’ve opened #728 (I thought there was already an issue but can’t find it); subscribe there for updates!

[DanTup]

I’ve tidied up what I’d started and pushed to the flutter branch (I’ve removed the flutter-launch-wip branch). The basics are working, though there’s something wonky with debugging/stepping atm though I’m out of time today. There’s a FlutterRun class which builds on the StdIoService that’s used for Analyzer+FlutterDaemon, though currently there’s a bunch of hacks that need tidying up. I’m gonna start adding the outstanding items to the top of this issue.

Restarting debug sessions is also currently broken (I’m awaiting a response from MS, I don’t understand why it doesn’t work; could be a Code bug) and there’s no hot reload (though that’s pretty trivial to add now).