Continue to dispatch CustomEvent instances in CommandRegistry

This changes the approach used to synthesize an event to dispatch. Since
we can no longer point to the original event as our prototype, we build
a new CustomEvent instance and override select properties with
Object.defineProperty. This preserves the contract with event handlers
by giving them a true CustomEvent instance.

src/command-registry.coffee

@@ -175,17 +175,8 @@ class CommandRegistry
   # * `commandName` {String} indicating the name of the command to dispatch.
   dispatch: (target, commandName, detail) ->
     event = new CustomEvent(commandName, {bubbles: true, detail})
-    eventWithTarget = Object.create {},
-      target: value: target
-      preventDefault: value: ->
-      stopPropagation: value: ->
-      stopImmediatePropagation: value: ->
-    # NOTE: In Chrome 43, Object.create doesn't work well with CustomEvent;
-    # However utilizing _.defaults here doesn't really do anything since all properties have been
-    # moved from "own" to the prototype, so we should update this to fully shadowing the properties
-    # from Event.prototype (+ detail property from CustomEvent).
-    eventWithTarget = _.defaults(eventWithTarget, event)
-    @handleCommandEvent(eventWithTarget)
+    Object.defineProperty(event, 'target', value: target)
+    @handleCommandEvent(event)
 
   # Public: Invoke the given callback before dispatching a command event.
   #
@@ -213,37 +204,36 @@ class CommandRegistry
       @selectorBasedListenersByCommandName[commandName] = listeners.slice()
     return
 
-  handleCommandEvent: (originalEvent) =>
+  handleCommandEvent: (event) =>
     propagationStopped = false
     immediatePropagationStopped = false
     matched = false
-    currentTarget = originalEvent.target
+    currentTarget = event.target
+    {preventDefault, stopPropagation, stopImmediatePropagation, abortKeyBinding} = event
 
-    syntheticEvent = Object.create {},
-      eventPhase: value: Event.BUBBLING_PHASE
-      currentTarget: get: -> currentTarget
-      target: value: currentTarget
-      preventDefault: value: ->
-        originalEvent.preventDefault()
-      stopPropagation: value: ->
-        originalEvent.stopPropagation()
+    dispatchedEvent = new CustomEvent(event.type, {bubbles: true, detail: event.detail})
+    Object.defineProperty dispatchedEvent, 'eventPhase', value: Event.BUBBLING_PHASE
+    Object.defineProperty dispatchedEvent, 'currentTarget', get: -> currentTarget
+    Object.defineProperty dispatchedEvent, 'target', value: currentTarget
+    Object.defineProperty dispatchedEvent, 'preventDefault', value: ->
+      event.preventDefault()
+    Object.defineProperty dispatchedEvent, 'stopPropagation', value: ->
+      event.stopPropagation()
       propagationStopped = true
-      stopImmediatePropagation: value: ->
-        originalEvent.stopImmediatePropagation()
+    Object.defineProperty dispatchedEvent, 'stopImmediatePropagation', value: ->
+      event.stopImmediatePropagation()
       propagationStopped = true
       immediatePropagationStopped = true
-      abortKeyBinding: value: ->
-        originalEvent.abortKeyBinding?()
-    # NOTE: See similar scenario in ::dispatch.
-    syntheticEvent = _.defaults(syntheticEvent, originalEvent)
+    Object.defineProperty dispatchedEvent, 'abortKeyBinding', value: ->
+      event.abortKeyBinding?()
 
-    @emitter.emit 'will-dispatch', syntheticEvent
+    @emitter.emit 'will-dispatch', dispatchedEvent
 
     loop
-      listeners = @inlineListenersByCommandName[originalEvent.type]?.get(currentTarget) ? []
+      listeners = @inlineListenersByCommandName[event.type]?.get(currentTarget) ? []
       if currentTarget.webkitMatchesSelector?
         selectorBasedListeners =
-          (@selectorBasedListenersByCommandName[originalEvent.type] ? [])
+          (@selectorBasedListenersByCommandName[event.type] ? [])
             .filter (listener) -> currentTarget.webkitMatchesSelector(listener.selector)
             .sort (a, b) -> a.compare(b)
         listeners = listeners.concat(selectorBasedListeners)
@@ -252,7 +242,7 @@ class CommandRegistry
 
       for listener in listeners
         break if immediatePropagationStopped
-        listener.callback.call(currentTarget, syntheticEvent)
+        listener.callback.call(currentTarget, dispatchedEvent)
 
       break if currentTarget is window
       break if propagationStopped