Use String.charAt for character by index lookup

Using object property accessor for string character lookup breaks in
non-modern browsers.

src/browser/ui/ReactMount.js

@@ -63,7 +63,7 @@ var findComponentRootReusableArray = [];
 function firstDifferenceIndex(string1, string2) {
   var minLen = Math.min(string1.length, string2.length);
   for (var i = 0; i < minLen; i++) {
-    if (string1[i] !== string2[i]) {
+    if (string1.charAt(i) !== string2.charAt(i)) {
       return i;
     }
   }