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

src/runtime.h

Index: src/runtime.h
diff --git a/src/runtime.h b/src/runtime.h
index c915cf38da4f9b1bd0d075d49391aaafb4fcde68..3056217ea6734c4e1575f7b0e0980bb6d660f019 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -197,6 +197,7 @@ namespace internal {
   F(StringLocaleCompare, 2, 1) \
   F(SubString, 3, 1) \
   F(StringReplaceRegExpWithString, 4, 1) \
+  F(StringReplaceOneCharWithString, 3, 1) \
   F(StringMatch, 3, 1) \
   F(StringTrim, 3, 1) \
   F(StringToArray, 2, 1) \
@@ -629,6 +630,12 @@ class Runtime : public AllStatic {
   // Get the intrinsic function with the given FunctionId.
   static const Function* FunctionForId(FunctionId id);
 
+  static Handle<String> StringReplaceOneCharWithString(Isolate* isolate,
+                                                       Handle<String> subject,
+                                                       Handle<String> search,
+                                                       Handle<String> replace,
+                                                       bool* found);
+
   // General-purpose helper functions for runtime system.
   static int StringMatch(Isolate* isolate,
                          Handle<String> sub,