Normalize ++x to x++. (#5660)

There are only 10 instances of ++x in our codebase, compared with over 500 instances of x++.  The stlye guide has no opinion on which to use, nor do I.  But the lack of consistency was making regex searches for bugs more difficult.

core/utils.js

@@ -418,7 +418,7 @@ const tokenizeInterpolation_ = function(message, parseInterpolationTokens) {
   // Merge adjacent text tokens into a single string.
   const mergedTokens = [];
   buffer.length = 0;
-  for (let i = 0; i < tokens.length; ++i) {
+  for (let i = 0; i < tokens.length; i++) {
     if (typeof tokens[i] === 'string') {
       buffer.push(tokens[i]);
     } else {