setBreakpointState method Null safety
setBreakpointState RPC allows for breakpoints to be enabled or
disabled, without requiring for the breakpoint to be completely removed.
isolateId refers to an isolate which has exited, then the
Sentinel is returned.
Breakpoint is the updated breakpoint with its new values.
Future<Breakpoint> setBreakpointState( String isolateId, String breakpointId, bool enable);