It should be possible to right-click on a file in the Explorer pane and choose "Create launch configuration" that will create a launch config:

• Either in the WorkspaceFolder or Workspace file (workspace file if there are already configs there, otherwise Workspace Folder)
• Named sensibly ("Dart (bin/foo.dart)" or "Flutter (lib/main.dart)")
• cwd set to the nearest project root
• program set as a relative path from cwd