If when is true, stop the program as if a breakpoint were hit at the
following statement.
Returns the value of when. Some debuggers may display message.
NOTE: When invoked, the isolate will not return until a debugger continues execution. When running in the Dart VM the behaviour is the same regardless of whether or not a debugger is connected. When compiled to JavaScript, this uses the "debugger" statement, and behaves exactly as that does.
Source
external bool debugger({bool when: true, String message});