email_validator_stream.dart

import 'dart:async';
import 'dart:html';

void main() {
  final InputElement input = querySelector('input');
  final DivElement div = querySelector('div');

  div.innerHtml = "Enter a valid email";

  final emailValidator =
      new StreamTransformer.fromHandlers(handleData: (input, sink) {
    if (input.contains("@"))
      sink.add(input);
    else
      sink.addError("Email must have an '@' symbol!");
  });

  final emailDotValidator =
      new StreamTransformer.fromHandlers(handleData: (input, sink) {
    if (input.contains("."))
      sink.add(input);
    else
      sink.addError("Email must have a '.' character!");
  });

  input.onInput
//       .take(4)
      .map((dynamic event) {
        return event.target.value;
      })
      .transform(emailValidator)
      .transform(emailDotValidator)
      .listen((inputValue) => div.innerHtml = '',
          onError: (err) => div.innerHtml = err);
}