lib: remove string_decoder.js var redeclarations

PR-URL: https://github.com/nodejs/node/pull/4978
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/string_decoder.js

@@ -96,7 +96,7 @@ StringDecoder.prototype.write = function(buffer) {
     charStr = charBuffer.toString(encoding, 0, charLength);
 
     // CESU-8: lead surrogate (D800-DBFF) is also the incomplete character
-    var charCode = charStr.charCodeAt(charStr.length - 1);
+    const charCode = charStr.charCodeAt(charStr.length - 1);
     if (charCode >= 0xD800 && charCode <= 0xDBFF) {
       charLength += surrogateSize;
       charStr = '';
@@ -128,8 +128,8 @@ StringDecoder.prototype.write = function(buffer) {
   this.charLength = charLength;
   charStr += buffer.toString(encoding, 0, end);
 
-  var end = charStr.length - 1;
+  end = charStr.length - 1;
-  var charCode = charStr.charCodeAt(end);
+  const charCode = charStr.charCodeAt(end);
   // CESU-8: lead surrogate (D800-DBFF) is also the incomplete character
   if (charCode >= 0xD800 && charCode <= 0xDBFF) {
     charLength += surrogateSize;