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

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 3608bac8facc279b0d0c31cd299cec641e58d714..e858b0fd526e7c8b8995981460bf7130b5974228 100644
--- a/src/string.js
+++ b/src/string.js
@@ -244,6 +244,11 @@ function StringReplace(search, replace) {
 
   // 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;