Freeze UTFSequence

Summary:
Don't want anyone accidentally mutating it.

Also make deepFreezeAndThrowOnMutationInDev easier to use with nice flow typing.

Reviewed By: yungsters

Differential Revision: D6974089

fbshipit-source-id: 0f90e7939cb726893fa353a4f2a6bbba701205bc

Libraries/UTFSequence.js

@@ -13,14 +13,16 @@
 
 'use strict';
 
+const deepFreezeAndThrowOnMutationInDev = require('deepFreezeAndThrowOnMutationInDev');
+
 /**
  * A collection of Unicode sequences for various characters and emoji.
  *
  *  - More explicit than using the sequences directly in code.
  *  - Source code should be limitted to ASCII.
  *  - Less chance of typos.
  */
-const UTFSequence = {
+const UTFSequence = deepFreezeAndThrowOnMutationInDev({
   MIDDOT: '\u00B7', // normal middle dot: ·
   MIDDOT_SP: '\u00A0\u00B7\u00A0', //  · 
   MIDDOT_KATAKANA: '\u30FB', // katakana middle dot
@@ -30,6 +32,6 @@ const UTFSequence = {
   NDASH_SP: '\u00A0\u2013\u00A0', //  – 
   NBSP: '\u00A0', // non-breaking space:  
   PIZZA: '\uD83C\uDF55',
-};
+});
 
 module.exports = UTFSequence;

Libraries/Utilities/deepFreezeAndThrowOnMutationInDev.js

@@ -29,13 +29,13 @@
  * Freezing the object and adding the throw mechanism is expensive and will
  * only be used in DEV.
  */
-function deepFreezeAndThrowOnMutationInDev(object: Object) {
+function deepFreezeAndThrowOnMutationInDev<T: Object>(object: T): T {
   if (__DEV__) {
     if (typeof object !== 'object' ||
         object === null ||
         Object.isFrozen(object) ||
         Object.isSealed(object)) {
-      return;
+      return object;
     }
 
     var keys = Object.keys(object);
@@ -58,6 +58,7 @@ function deepFreezeAndThrowOnMutationInDev(object: Object) {
       }
     }
   }
+  return object;
 }
 
 function throwOnImmutableMutation(key, value) {