AsyncSnapshot<T>.withError  constructor 
    Null safety
- ConnectionState state,
- Object error,
- [StackTrace stackTrace = StackTrace.empty]
Creates an AsyncSnapshot in the specified state with the specified error
and a stackTrace.
If no stackTrace is explicitly specified, StackTrace.empty will be used instead.
Implementation
const AsyncSnapshot.withError(
  ConnectionState state,
  Object error, [
  StackTrace stackTrace = StackTrace.empty,
]) : this._(state, null, error, stackTrace);