Dependency fix and some style fixes.
Every getter of a custom element that has the signature onEventName is assumed to be an "event member".
Reworked gen_html_setters.dart to work with the current DartSDK version (r23200).
Added generation of event lists for the native HTML elements.
Note: xtag is no longer in the field list of Element.
Events are checked against the event lists of the native elements, instead of searching events in the custom element members.
Fixed failing test.