A MarkedValueList is basically a Vector<JS::Value> that registers with
the Heap and makes sure that the stored values don't get GC'd.
Before this change, we were unsafely keeping Vector<JS::Value> in some
places, which is out-of-reach for the live reference finding logic
since Vector puts its elements on the heap by default.
We now pass all the JavaScript tests even when running with "js -g",
which does a GC on every heap allocation.
This patch adds the Event base class, along with a MouseEvent subclass.
We now dispatch MouseEvent objects for mousedown, mouseup and mousemove
and these objects have the .offsetX and .offsetY properties.
Both of those properties are hard-coded at the moment. This will be
fixed in the next patch. :^)
This patch adds the EventTarget class and makes Node inherit from it.
You can register event listeners on an EventTarget, and when you call
dispatch_event() on it, the event listeners will get invoked.
An event listener is basically a wrapper around a JS::Function*.
This is pretty far from how DOM events should eventually work, but it's
a place to start and we'll build more on top of this. :^)
