diff --git a/Libraries/Utilities/PerformanceLogger.js b/Libraries/Utilities/PerformanceLogger.js
index e0f0ccbe3..451b34415 100644
--- a/Libraries/Utilities/PerformanceLogger.js
+++ b/Libraries/Utilities/PerformanceLogger.js
@@ -7,16 +7,25 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule PerformanceLogger
+ * @flow
  */
 'use strict';
 
 const BatchedBridge = require('BatchedBridge');
-const performanceNow = global.nativePerformanceNow || require('fbjs/lib/performanceNow');
 const Systrace = require('Systrace');
 
-var timespans = {};
-var extras = {};
-var cookies = {};
+const performanceNow = global.nativePerformanceNow || require('fbjs/lib/performanceNow');
+
+type Timespan = {
+  description?: string,
+  totalTime?: number,
+  startTime?: number,
+  endTime?: number,
+};
+
+let timespans: {[key:string]: Timespan} = {};
+let extras: {[key:string]: any} = {};
+const cookies: {[key:string]: number} = {};
 
 const PRINT_TO_CONSOLE = false;
 
@@ -24,8 +33,8 @@ const PRINT_TO_CONSOLE = false;
  * This is meant to collect and log performance data in production, which means
  * it needs to have minimal overhead.
  */
-var PerformanceLogger = {
-  addTimespan(key, lengthInMs, description) {
+const PerformanceLogger = {
+  addTimespan(key: string, lengthInMs: number, description?: string) {
     if (timespans[key]) {
       if (__DEV__) {
         console.log(
@@ -42,7 +51,7 @@ var PerformanceLogger = {
     };
   },
 
-  startTimespan(key, description) {
+  startTimespan(key: string, description?: string) {
     if (timespans[key]) {
       if (__DEV__) {
         console.log(
@@ -63,8 +72,9 @@ var PerformanceLogger = {
     }
   },
 
-  stopTimespan(key) {
-    if (!timespans[key] || !timespans[key].startTime) {
+  stopTimespan(key: string) {
+    const timespan = timespans[key];
+    if (!timespan || !timespan.startTime) {
       if (__DEV__) {
         console.log(
           'PerformanceLogger: Attempting to end a timespan that has not started ',
@@ -73,7 +83,7 @@ var PerformanceLogger = {
       }
       return;
     }
-    if (timespans[key].endTime) {
+    if (timespan.endTime) {
       if (__DEV__) {
         console.log(
           'PerformanceLogger: Attempting to end a timespan that has already ended ',
@@ -83,14 +93,14 @@ var PerformanceLogger = {
       return;
     }
 
+    timespan.endTime = performanceNow();
+    timespan.totalTime = timespan.endTime - (timespan.startTime || 0);
     if (__DEV__ && PRINT_TO_CONSOLE) {
       console.log('PerformanceLogger.js', 'end: ' + key);
     }
+
     Systrace.endAsyncEvent(key, cookies[key]);
     delete cookies[key];
-    timespans[key].endTime = performanceNow();
-    timespans[key].totalTime =
-      timespans[key].endTime - timespans[key].startTime;
   },
 
   clear() {
@@ -99,7 +109,7 @@ var PerformanceLogger = {
   },
 
   clearCompleted() {
-    for (var key in timespans) {
+    for (const key in timespans) {
       if (timespans[key].totalTime) {
         delete timespans[key];
       }
@@ -107,7 +117,7 @@ var PerformanceLogger = {
     extras = {};
   },
 
-  clearExceptTimespans(keys) {
+  clearExceptTimespans(keys: Array<string>) {
     timespans = Object.keys(timespans).reduce(function(previous, key) {
       if (keys.indexOf(key) !== -1) {
         previous[key] = timespans[key];
@@ -125,21 +135,21 @@ var PerformanceLogger = {
     return timespans;
   },
 
-  hasTimespan(key) {
+  hasTimespan(key: string) {
     return !!timespans[key];
   },
 
   logTimespans() {
-    for (var key in timespans) {
+    for (const key in timespans) {
       if (timespans[key].totalTime) {
         console.log(key + ': ' + timespans[key].totalTime + 'ms');
       }
     }
   },
 
-  addTimespans(newTimespans, labels) {
-    for (var i = 0, l = newTimespans.length; i < l; i += 2) {
-      var label = labels[i / 2];
+  addTimespans(newTimespans: Array<number>, labels: Array<string>) {
+    for (let i = 0, l = newTimespans.length; i < l; i += 2) {
+      const label = labels[i / 2];
       PerformanceLogger.addTimespan(
         label,
         (newTimespans[i + 1] - newTimespans[i]),
@@ -148,7 +158,7 @@ var PerformanceLogger = {
     }
   },
 
-  setExtra(key, value) {
+  setExtra(key: string, value: any) {
     if (extras[key]) {
       if (__DEV__) {
         console.log(