Remove EventTarget, and subsequent fallout.

The primary goal of this change was to remove EventTarget from the
sky_engine C++ code. Since EventTarget is so core to the entire event
system that sky_engine was based on, this is a rather invasive change.
As such, it had some knock-on effects. I deleted some of the files
that were affected, and cauterised the remainder.

In many cases, a file would depend on another file that it didn't
include directly, but instead included indirectly via another file
that I deleted. When this happened, if the features that this broke
were obsolete, I sometimes just removed the features instead.

Specifically:
- removed EventTarget
- removed EventQueue, since without a target, what's a queue going to
  do?
- same with EventDispatch*
- removed ExecutionContext, since it had an EventQueue and nothing
  else it did was relevant to Sky anymore
- removed ActiveDOMObject, which was all about ExecutionContexts
- removed ContextLifecycleNotifier since it dependend on
  ExecutionContext and ActiveDOMObject
- removed the other Lifecycle classes for consistency, and replaced
  them with four booleans in the Document class
- removed some of the attributes that are no longer relevant from
  IDLExtendedAttributes (ConstructorCallWith and
  CallWith=ExecutionContext)
- removed the Document member on DOMDartState since we never set it to
  anything but null.
- removed BuiltinSky::InstallWindow since it relied on the Document
  member of DOMDartState
- removed EventHandler, EventListener, and mentions of those in
  various binding scripts
- removed NewEventHandler, since we're not using that either
- removed the following interfaces from the Sky Dart API:
  - EventTarget
  - EventListener (since without a target, there's no way to listen)
  - FocusEvent (since it's only member was an EventTarget)
  - HashChangeEvent (mostly by accident, but it's defunct anyway)
  - FontFace (it used ConstructorCallWith=ExecutionContext)
- changed the following interfaces of the Sky DART API:
  - MediaQueryList is no longer an EventTarget
  - Node is no longer an EventTarget
  - Document no longer has defaultView (depended on
    DOMDartState's document)
  - DocumentFragment, Element, Range, and Text no longer have a
    constructor (they all depended on DOMDartState's document, which
    is now gone)
  - Event lost its EventTarget members and path.
  - Window lost its WindowTimers partial interface (it used
    EventTarget and ExecutionContext a lot)
- removed numerous hacks in the bindings around features that are now
  gone, like addEventListener
- removed a bunch of console logging code, since that relied on
  ExecutionContext
- cauterised the wound in FontFace.cpp by removing constructors and
  methods that called now-removed features
- same with MediaQuery and friends
- same with some editor features and focus-related features
- same with Document
- removed DOMTimer classes since they use ExecutionContexts

packages/flutter/lib/widgets/widget.dart

@@ -334,7 +334,7 @@ class Listener extends TagNode  {
     PointerEventListener onPointerDown,
     PointerEventListener onPointerMove,
     PointerEventListener onPointerUp,
-    Map<String, sky.EventListener> custom
+    Map<String, EventListener> custom
   }) : listeners = _createListeners(
          onWheel: onWheel,
          onGestureFlingCancel: onGestureFlingCancel,
@@ -351,9 +351,9 @@ class Listener extends TagNode  {
        ),
        super(child, key: key);
 
-  final Map<String, sky.EventListener> listeners;
+  final Map<String, EventListener> listeners;
 
-  static Map<String, sky.EventListener> _createListeners({
+  static Map<String, EventListener> _createListeners({
     EventListener onWheel,
     GestureEventListener onGestureFlingCancel,
     GestureEventListener onGestureFlingStart,
@@ -365,11 +365,11 @@ class Listener extends TagNode  {
     PointerEventListener onPointerDown,
     PointerEventListener onPointerMove,
     PointerEventListener onPointerUp,
-    Map<String, sky.EventListener> custom
+    Map<String, EventListener> custom
   }) {
     var listeners = custom != null ?
-        new HashMap<String, sky.EventListener>.from(custom) :
-        new HashMap<String, sky.EventListener>();
+        new HashMap<String, EventListener>.from(custom) :
+        new HashMap<String, EventListener>();
 
     if (onWheel != null)
       listeners['wheel'] = onWheel;
@@ -398,7 +398,7 @@ class Listener extends TagNode  {
   }
 
   void _handleEvent(sky.Event e) {
-    sky.EventListener listener = listeners[e.type];
+    EventListener listener = listeners[e.type];
     if (listener != null) {
       listener(e);
     }