diff --git a/packages/react-dom/src/events/DOMPluginEventSystem.js b/packages/react-dom/src/events/DOMPluginEventSystem.js
index b11807fdca3e6..eea047b959879 100644
--- a/packages/react-dom/src/events/DOMPluginEventSystem.js
+++ b/packages/react-dom/src/events/DOMPluginEventSystem.js
@@ -15,7 +15,10 @@ import {
   SHOULD_NOT_PROCESS_POLYFILL_EVENT_PLUGINS,
 } from './EventSystemFlags';
 import type {AnyNativeEvent} from './PluginModuleType';
-import type {ReactSyntheticEvent} from './ReactSyntheticEventType';
+import type {
+  KnownReactSyntheticEvent,
+  ReactSyntheticEvent,
+} from './ReactSyntheticEventType';
 import type {ElementListenerMapEntry} from '../client/ReactDOMComponentTree';
 import type {EventPriority} from 'shared/ReactTypes';
 import type {Fiber} from 'react-reconciler/src/ReactInternalTypes';
@@ -917,15 +920,12 @@ function getLowestCommonAncestor(instA: Fiber, instB: Fiber): Fiber | null {
 
 function accumulateEnterLeaveListenersForEvent(
   dispatchQueue: DispatchQueue,
-  event: ReactSyntheticEvent,
+  event: KnownReactSyntheticEvent,
   target: Fiber,
   common: Fiber | null,
   inCapturePhase: boolean,
 ): void {
   const registrationName = event._reactName;
-  if (registrationName === undefined) {
-    return;
-  }
   const listeners: Array<DispatchListener> = [];
 
   let instance = target;
@@ -969,8 +969,8 @@ function accumulateEnterLeaveListenersForEvent(
 // phase event listeners.
 export function accumulateEnterLeaveTwoPhaseListeners(
   dispatchQueue: DispatchQueue,
-  leaveEvent: ReactSyntheticEvent,
-  enterEvent: null | ReactSyntheticEvent,
+  leaveEvent: KnownReactSyntheticEvent,
+  enterEvent: null | KnownReactSyntheticEvent,
   from: Fiber | null,
   to: Fiber | null,
 ): void {
diff --git a/packages/react-dom/src/events/ReactSyntheticEventType.js b/packages/react-dom/src/events/ReactSyntheticEventType.js
index e37af02d162d3..853aa3a6aefa7 100644
--- a/packages/react-dom/src/events/ReactSyntheticEventType.js
+++ b/packages/react-dom/src/events/ReactSyntheticEventType.js
@@ -22,14 +22,26 @@ export type DispatchConfig = {|
   eventPriority?: EventPriority,
 |};
 
-export type ReactSyntheticEvent = {|
+type BaseSyntheticEvent = {
   isPersistent: () => boolean,
   isPropagationStopped: () => boolean,
   _dispatchInstances?: null | Array<Fiber | null> | Fiber,
   _dispatchListeners?: null | Array<Function> | Function,
-  _reactName: string,
   _targetInst: Fiber,
   nativeEvent: Event,
+  target?: mixed,
+  relatedTarget?: mixed,
   type: string,
   currentTarget: null | EventTarget,
-|};
+};
+
+export type KnownReactSyntheticEvent = BaseSyntheticEvent & {
+  _reactName: string,
+};
+export type UnknownReactSyntheticEvent = BaseSyntheticEvent & {
+  _reactName: null,
+};
+
+export type ReactSyntheticEvent =
+  | KnownReactSyntheticEvent
+  | UnknownReactSyntheticEvent;
diff --git a/packages/react-dom/src/events/SyntheticEvent.js b/packages/react-dom/src/events/SyntheticEvent.js
index e226e95833cae..01742c705278c 100644
--- a/packages/react-dom/src/events/SyntheticEvent.js
+++ b/packages/react-dom/src/events/SyntheticEvent.js
@@ -3,17 +3,23 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 /* eslint valid-typeof: 0 */
 
 import getEventCharCode from './getEventCharCode';
 
+type EventInterfaceType = {
+  [propName: string]: 0 | ((event: {[propName: string]: mixed}) => mixed),
+};
+
 /**
  * @interface Event
  * @see http://www.w3.org/TR/DOM-Level-3-Events/
  */
-const EventInterface = {
+const EventInterface: EventInterfaceType = {
   eventPhase: 0,
   bubbles: 0,
   cancelable: 0,
@@ -46,12 +52,12 @@ function functionThatReturnsFalse() {
  * DOM interface; custom application-specific events can also subclass this.
  */
 export function SyntheticEvent(
-  reactName,
-  reactEventType,
-  targetInst,
-  nativeEvent,
-  nativeEventTarget,
-  Interface = EventInterface,
+  reactName: string | null,
+  reactEventType: string,
+  targetInst: Fiber,
+  nativeEvent: {[propName: string]: mixed},
+  nativeEventTarget: null | EventTarget,
+  Interface: EventInterfaceType = EventInterface,
 ) {
   this._reactName = reactName;
   this._targetInst = targetInst;
@@ -95,6 +101,7 @@ Object.assign(SyntheticEvent.prototype, {
 
     if (event.preventDefault) {
       event.preventDefault();
+      // $FlowFixMe - flow is not aware of `unknown` in IE
     } else if (typeof event.returnValue !== 'unknown') {
       event.returnValue = false;
     }
@@ -109,6 +116,7 @@ Object.assign(SyntheticEvent.prototype, {
 
     if (event.stopPropagation) {
       event.stopPropagation();
+      // $FlowFixMe - flow is not aware of `unknown` in IE
     } else if (typeof event.cancelBubble !== 'unknown') {
       // The ChangeEventPlugin registers a "propertychange" event for
       // IE. This event does not support bubbling or cancelling, and
@@ -138,7 +146,7 @@ Object.assign(SyntheticEvent.prototype, {
   isPersistent: functionThatReturnsTrue,
 });
 
-export const UIEventInterface = {
+export const UIEventInterface: EventInterfaceType = {
   ...EventInterface,
   view: 0,
   detail: 0,
@@ -154,7 +162,7 @@ let isMovementYSet = false;
  * @interface MouseEvent
  * @see http://www.w3.org/TR/DOM-Level-3-Events/
  */
-export const MouseEventInterface = {
+export const MouseEventInterface: EventInterfaceType = {
   ...UIEventInterface,
   screenX: 0,
   screenY: 0,
@@ -213,7 +221,7 @@ export const MouseEventInterface = {
  * @interface DragEvent
  * @see http://www.w3.org/TR/DOM-Level-3-Events/
  */
-export const DragEventInterface = {
+export const DragEventInterface: EventInterfaceType = {
   ...MouseEventInterface,
   dataTransfer: 0,
 };
@@ -222,7 +230,7 @@ export const DragEventInterface = {
  * @interface FocusEvent
  * @see http://www.w3.org/TR/DOM-Level-3-Events/
  */
-export const FocusEventInterface = {
+export const FocusEventInterface: EventInterfaceType = {
   ...UIEventInterface,
   relatedTarget: 0,
 };
@@ -232,7 +240,7 @@ export const FocusEventInterface = {
  * @see http://www.w3.org/TR/css3-animations/#AnimationEvent-interface
  * @see https://developer.mozilla.org/en-US/docs/Web/API/AnimationEvent
  */
-export const AnimationEventInterface = {
+export const AnimationEventInterface: EventInterfaceType = {
   ...EventInterface,
   animationName: 0,
   elapsedTime: 0,
@@ -243,7 +251,7 @@ export const AnimationEventInterface = {
  * @interface Event
  * @see http://www.w3.org/TR/clipboard-apis/
  */
-export const ClipboardEventInterface = {
+export const ClipboardEventInterface: EventInterfaceType = {
   ...EventInterface,
   clipboardData: function(event) {
     return 'clipboardData' in event
@@ -256,7 +264,7 @@ export const ClipboardEventInterface = {
  * @interface Event
  * @see http://www.w3.org/TR/DOM-Level-3-Events/#events-compositionevents
  */
-export const CompositionEventInterface = {
+export const CompositionEventInterface: EventInterfaceType = {
   ...EventInterface,
   data: 0,
 };
@@ -267,7 +275,7 @@ export const CompositionEventInterface = {
  *      /#events-inputevents
  */
 // Happens to share the same list for now.
-export const InputEventInterface = CompositionEventInterface;
+export const InputEventInterface: EventInterfaceType = CompositionEventInterface;
 
 /**
  * Normalization of deprecated HTML5 `key` values
diff --git a/packages/react-dom/src/events/plugins/EnterLeaveEventPlugin.js b/packages/react-dom/src/events/plugins/EnterLeaveEventPlugin.js
index 32ab29a91042b..196d99c711997 100644
--- a/packages/react-dom/src/events/plugins/EnterLeaveEventPlugin.js
+++ b/packages/react-dom/src/events/plugins/EnterLeaveEventPlugin.js
@@ -25,6 +25,7 @@ import {
   isContainerMarkedAsRoot,
 } from '../../client/ReactDOMComponentTree';
 import {accumulateEnterLeaveTwoPhaseListeners} from '../DOMPluginEventSystem';
+import type {KnownReactSyntheticEvent} from '../ReactSyntheticEventType';
 
 import {HostComponent, HostText} from 'react-reconciler/src/ReactWorkTags';
 import {getNearestMountedFiber} from 'react-reconciler/src/ReactFiberTreeReflection';
@@ -147,23 +148,23 @@ function extractEvents(
   leave.target = fromNode;
   leave.relatedTarget = toNode;
 
-  let enter = new SyntheticEvent(
-    enterEventType,
-    eventTypePrefix + 'enter',
-    to,
-    nativeEvent,
-    nativeEventTarget,
-    eventInterface,
-  );
-  enter.target = toNode;
-  enter.relatedTarget = fromNode;
+  let enter: KnownReactSyntheticEvent | null = null;
 
-  // If we are not processing the first ancestor, then we
-  // should not process the same nativeEvent again, as we
-  // will have already processed it in the first ancestor.
+  // We should only process this nativeEvent if we are processing
+  // the first ancestor. Next time, we will ignore the event.
   const nativeTargetInst = getClosestInstanceFromNode((nativeEventTarget: any));
-  if (nativeTargetInst !== targetInst) {
-    enter = null;
+  if (nativeTargetInst === targetInst) {
+    const enterEvent: KnownReactSyntheticEvent = new SyntheticEvent(
+      enterEventType,
+      eventTypePrefix + 'enter',
+      to,
+      nativeEvent,
+      nativeEventTarget,
+      eventInterface,
+    );
+    enterEvent.target = toNode;
+    enterEvent.relatedTarget = fromNode;
+    enter = enterEvent;
   }
 
   accumulateEnterLeaveTwoPhaseListeners(dispatchQueue, leave, enter, from, to);