Support concurrent debugging of Dart/Flutter apps

[DanTup]

Eg. you might want to run a server-side Dart web API app and a frontend app (or Flutter app). This requires multi-root support but will also need code that assumes a single debug session to be fixed, like this:

Dart-Code/src/commands/debug.ts

Line 40 in 85a9b91

this.observatoryUri = e.body.observatoryUri;