Debug -> Start Debugging doesn't work without a launch.json

[DanTup]

If I create a .dart file and install the Dart extension, I can debug it if I create a launch.json file like this:

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Dart",
      "request": "launch",
      "type": "dart"
    }
  ]
}

However if there is no launch.json, then Start -> Start Debugging does nothing (it doesn't even show a message to indicate why).

In VS Code, when there's no launch.json, VS Code will use the open file to decide which debugger to use (and if there's no open file, it will prompt the user to pick from a list of all debuggers). This is generally how we encourage Dart/Flutter users to run (only creating a launch.json if they need to customise something).

[vince-fugnitto]

@DanTup I have a partial fix #6490. More effort will be required to prompt users for different debug types through the quick-open however.

[DanTup]

Ok great. FWIW, VS Code doesn't just create a launch.json, it can launch without one (which is preferred, because otherwise people have to maintain it). Whether it creates one depends on the return value from resolveDebugConfiguration (null causes launch.json to be created and opened, undefined is silent, and anything else will launch - see https://github.com/Dart-Code/Dart-Code/blob/master/src/extension/providers/debug_config_provider.ts#L101 and other return statements in there).