[builtins] Migrate String.prototype.charCodeAt and String.prototype.charAt to TurboFan.

src/runtime/runtime-strings.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/runtime/runtime-utils.h"
 
 #include "src/arguments.h"
 #include "src/conversions-inl.h"
 #include "src/isolate-inl.h"
 #include "src/regexp/jsregexp-inl.h"
@@ skipping 1255 matching lines @@
   SealHandleScope shs(isolate);
   DCHECK(args.length() == 2);
   if (!args[0]->IsString()) return Smi::FromInt(0);
   if (!args[1]->IsNumber()) return Smi::FromInt(0);
   if (std::isinf(args.number_at(1))) return isolate->heap()->empty_string();
   Object* code = __RT_impl_Runtime_StringCharCodeAtRT(args, isolate);
   if (code->IsNaN()) return isolate->heap()->empty_string();
   return __RT_impl_Runtime_StringCharFromCode(Arguments(1, &code), isolate);
 }
 
+RUNTIME_FUNCTION(Runtime_ExternalStringGetChar) {
+  SealHandleScope shs(isolate);
+  DCHECK_EQ(2, args.length());
+  CONVERT_ARG_CHECKED(ExternalString, string, 0);
+  CONVERT_INT32_ARG_CHECKED(index, 1);
+  return Smi::FromInt(string->Get(index));
+}
 
 RUNTIME_FUNCTION(Runtime_OneByteSeqStringGetChar) {
   SealHandleScope shs(isolate);
   DCHECK(args.length() == 2);
   CONVERT_ARG_CHECKED(SeqOneByteString, string, 0);
   CONVERT_INT32_ARG_CHECKED(index, 1);
   return Smi::FromInt(string->SeqOneByteStringGet(index));
 }
 
 
@@ skipping 32 matching lines @@
   SealHandleScope shs(isolate);
   DCHECK(args.length() == 2);
   if (!args[0]->IsString()) return isolate->heap()->undefined_value();
   if (!args[1]->IsNumber()) return isolate->heap()->undefined_value();
   if (std::isinf(args.number_at(1))) return isolate->heap()->nan_value();
   return __RT_impl_Runtime_StringCharCodeAtRT(args, isolate);
 }
 
 }  // namespace internal
 }  // namespace v8