listen method
Adds a subscription to this stream.
Returns a StreamSubscription which handles events from the stream using
the provided onData, onError and onDone handlers.
The handlers can be changed on the subscription, but they start out
as the provided functions.
On each data event from this stream, the subscriber's onData handler
is called. If onData is null, nothing happens.
On errors from this stream, the onError handler is called with the
error object and possibly a stack trace.
The onError callback must be of type void onError(error) or
void onError(error, StackTrace stackTrace). If onError accepts
two arguments it is called with the error object and the stack trace
(which could be null if the stream itself received an error without
stack trace).
Otherwise it is called with just the error object.
If onError is omitted, any errors on the stream are considered unhandled,
and will be passed to the current Zone's error handler.
By default unhandled async errors are treated
as if they were uncaught top-level errors.
If this stream closes and sends a done event, the onDone handler is
called. If onDone is null, nothing happens.
If cancelOnError is true, the subscription is automatically canceled
when the first error event is delivered. The default is false.
While a subscription is paused, or when it has been canceled, the subscription doesn't receive events and none of the event handler functions are called.
Implementation
StreamSubscription<T> listen(void onData(T event),
{Function onError, void onDone(), bool cancelOnError});