Fast path for string.replace that replaces a single character by a string.

src/string.js

 // Copyright 2006-2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 226 matching lines @@
     } else {
       return %StringReplaceRegExpWithString(subject,
                                             search,
                                             TO_STRING_INLINE(replace),
                                             lastMatchInfo);
     }
   }
 
   // Convert the search argument to a string and search for it.
   search = TO_STRING_INLINE(search);
+  if (search.length == 1 &&
+      IS_STRING(replace) &&
+      %StringIndexOf(replace, '$', 0) < 0) {
+    return %StringReplaceOneCharWithString(subject, search, replace);
+  }
   var start = %StringIndexOf(subject, search, 0);
   if (start < 0) return subject;
   var end = start + search.length;
 
   var builder = new ReplaceResultBuilder(subject);
   // prefix
   builder.addSpecialSlice(0, start);
 
   // Compute the string to replace with.
   if (IS_SPEC_FUNCTION(replace)) {
@@ skipping 742 matching lines @@
     "fixed", StringFixed,
     "italics", StringItalics,
     "small", StringSmall,
     "strike", StringStrike,
     "sub", StringSub,
     "sup", StringSup
   ));
 }
 
 SetUpString();