Add notifyListener flag to webdriver.dart.
Add notifyListener flag to webdriver.dart.

Allows toggling recording of WebDriverCommandEvents on or off. Useful when using a WebDriver instance to perform custom logging (screenshots, executing Javascript, etc.)
google/webdriver.dart,bwu-dart-contributing/webdriver.dart,staats-google/webdriver.dart,google/webdriver.dart,DrMarcII/webdriver.dart,DrMarcII/webdriver.dart,google/webdriver.dart,staats-google/webdriver.dart,DrMarcII/webdriver.dart,staats-google/webdriver.dart,DrMarcII/webdriver.dart,bwu-dart-contributing/webdriver.dart,bwu-dart-contributing/webdriver.dart,staats-google/webdriver.dart,bwu-dart-contributing/webdriver.dart