Extract event listener from memoizedProps on Fiber instances

This makes some basic events work with Fiber.

This is however not a complete solution since we may be reading the wrong
Fiber.

scripts/fiber/tests-failing.txt

@@ -137,9 +137,6 @@ src/renderers/dom/shared/eventPlugins/__tests__/ChangeEventPlugin-test.js
 * should listen for both change and input events when supported
 * should only fire events when the value changes for range inputs
 
-src/renderers/dom/shared/eventPlugins/__tests__/EnterLeaveEventPlugin-test.js
-* should set relatedTarget properly in iframe
-
 src/renderers/dom/shared/eventPlugins/__tests__/SelectEventPlugin-test.js
 * should skip extraction if no listeners are present
 * should extract if an `onSelect` listener is present

scripts/fiber/tests-passing.txt

@@ -605,6 +605,9 @@ src/renderers/dom/shared/eventPlugins/__tests__/BeforeInputEventPlugin-test.js
 src/renderers/dom/shared/eventPlugins/__tests__/ChangeEventPlugin-test.js
 * should unmount
 
+src/renderers/dom/shared/eventPlugins/__tests__/EnterLeaveEventPlugin-test.js
+* should set relatedTarget properly in iframe
+
 src/renderers/dom/shared/eventPlugins/__tests__/FallbackCompositionState-test.js
 * extracts value via `getText()`
 * extracts when inserted at start of text

src/renderers/shared/shared/event/EventPluginHub.js

@@ -96,7 +96,14 @@ var EventPluginHub = {
    * @return {?function} The stored callback.
    */
   getListener: function(inst, registrationName) {
-    var listener = inst._currentElement.props[registrationName];
+    var listener;
+    if (typeof inst.tag === 'number') {
+      // TODO: This is not safe because we might want the *other* Fiber's
+      // props depending on which is the current one.
+      listener = inst.memoizedProps[registrationName];
+    } else {
+      listener = inst._currentElement.props[registrationName];
+    }
     invariant(
       !listener || typeof listener === 'function',
       'Expected %s listener to be a function, instead got type %s',