[builtins] Move StringIndexOf to a builtin.

src/runtime/runtime-strings.cc

Index: src/runtime/runtime-strings.cc
diff --git a/src/runtime/runtime-strings.cc b/src/runtime/runtime-strings.cc
index f5bda59b26fe9316b02aecdd80b3b60742876a8b..6a6f14a65b25e0a3aa75b46d5e12d73e11d500bb 100644
--- a/src/runtime/runtime-strings.cc
+++ b/src/runtime/runtime-strings.cc
@@ -43,16 +43,21 @@ MaybeHandle<String> StringReplaceOneCharWithString(
 
     return subject;
   } else {
-    int index = String::IndexOf(isolate, subject, search, 0);
-    if (index == -1) return subject;
+    Object* index = String::IndexOf(isolate, subject, search,
+                                    isolate->factory()->undefined_value());
+    int index_int = -1;
+    index->ToInt32(&index_int);
+
+    if (index_int == -1) return subject;
     *found = true;
-    Handle<String> first = isolate->factory()->NewSubString(subject, 0, index);
+    Handle<String> first =
+        isolate->factory()->NewSubString(subject, 0, index_int);
     Handle<String> cons1;
     ASSIGN_RETURN_ON_EXCEPTION(
         isolate, cons1, isolate->factory()->NewConsString(first, replace),
         String);
-    Handle<String> second =
-        isolate->factory()->NewSubString(subject, index + 1, subject->length());
+    Handle<String> second = isolate->factory()->NewSubString(
+        subject, index_int + 1, subject->length());
     return isolate->factory()->NewConsString(cons1, second);
   }
 }
@@ -90,17 +95,8 @@ RUNTIME_FUNCTION(Runtime_StringReplaceOneCharWithString) {
 RUNTIME_FUNCTION(Runtime_StringIndexOf) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 3);
-
-  CONVERT_ARG_HANDLE_CHECKED(String, sub, 0);
-  CONVERT_ARG_HANDLE_CHECKED(String, pat, 1);
-  CONVERT_ARG_HANDLE_CHECKED(Object, index, 2);
-
-  uint32_t start_index = 0;
-  if (!index->ToArrayIndex(&start_index)) return Smi::FromInt(-1);
-
-  CHECK(start_index <= static_cast<uint32_t>(sub->length()));
-  int position = String::IndexOf(isolate, sub, pat, start_index);
-  return Smi::FromInt(position);
+  return String::IndexOf(isolate, args.at<Object>(0), args.at<Object>(1),
+                         args.at<Object>(2));
 }
 
 RUNTIME_FUNCTION(Runtime_StringLastIndexOf) {