[builtins] Move StringIndexOf to a builtin.

src/builtins/builtins-string.cc

Index: src/builtins/builtins-string.cc
diff --git a/src/builtins/builtins-string.cc b/src/builtins/builtins-string.cc
index c09a6b71f77514e33d48af3d82d7acb8882e0dae..b07123c6ca4f6a71b9781dc30b199f02c51e2bdd 100644
--- a/src/builtins/builtins-string.cc
+++ b/src/builtins/builtins-string.cc
@@ -477,6 +477,15 @@ void Builtins::Generate_StringPrototypeCharCodeAt(
   assembler->Return(result);
 }
 
+// ES6 section 21.1.3.8 String.prototype.indexOf ( searchString [ , position ] )
+BUILTIN(StringPrototypeIndexOf) {
+  HandleScope handle_scope(isolate);
+
+  return String::IndexOf(isolate, args.receiver(),
+                         args.atOrUndefined(isolate, 1),
+                         args.atOrUndefined(isolate, 2));
+}
+
 // ES6 section 21.1.3.9
 // String.prototype.lastIndexOf ( searchString [ , position ] )
 BUILTIN(StringPrototypeLastIndexOf) {