html / svg: shadowing definitions of onClick etc

[rakudrama]

Element and SvgElement both declare an onClick getter:

class Element extends Node ... {
...
  static const EventStreamProvider<MouseEvent> clickEvent =
      const EventStreamProvider<MouseEvent>('click');
...
  ElementStream<MouseEvent> get onClick => clickEvent.forElement(this);

class SvgElement extends Element implements GlobalEventHandlers {
...
  static const EventStreamProvider<MouseEvent> clickEvent =
      const EventStreamProvider<MouseEvent>('click');
...
  ElementStream<MouseEvent> get onClick => clickEvent.forElement(this);

These declarations are the same, so it would me more efficient if SvgElement inherited the onClick getter from Element.
With inheritance, there would be one declaration, which would allow get:onClick to be inlined (on dart2js).

We know the inlining is beneficial: onClick can currently be inlined when the receiver is known to be HtmlElement, and the inlined code avoids an allocation.