Generate getters for events

[sigmundch]

See dart-archive/core-elements#79

quote:

like

  async.Stream<dom.CustomEvent> get onPolymerActivate =>
      PolymerSelector._onPolymerActivate.forTarget(this);

  static const dom.EventStreamProvider<dom.CustomEvent> _onPolymerActivate =
      const dom.EventStreamProvider<dom.CustomEvent>('polymer-activate');

see also this discussion
https://groups.google.com/a/dartlang.org/forum/#!topic/web/_FDeF95uj4g

for a complete example see https://github.com/bwu-dart/polymer_elements/blob/master/lib/polymer_selector/polymer_selector.dart#L210-L214

[sigmundch]

see also dart-archive/core-elements#16

[jakemac53]

Lets revive this since events are often specified for elements now, and I believe hydroysis already parses them for us :)