[cleanup] The %StringAdd runtime function is always called on Strings.

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.h"
 #include "src/counters.h"
 #include "src/objects-inl.h"
@@ skipping 173 matching lines @@
   }
   isolate->counters()->sub_string_runtime()->Increment();
 
   return *isolate->factory()->NewSubString(string, start, end);
 }
 
 
 RUNTIME_FUNCTION(Runtime_StringAdd) {
   HandleScope scope(isolate);
   DCHECK_EQ(2, args.length());
-  CONVERT_ARG_HANDLE_CHECKED(Object, obj1, 0);
-  CONVERT_ARG_HANDLE_CHECKED(Object, obj2, 1);
+  CONVERT_ARG_HANDLE_CHECKED(String, str1, 0);
+  CONVERT_ARG_HANDLE_CHECKED(String, str2, 1);
   isolate->counters()->string_add_runtime()->Increment();
-  MaybeHandle<String> maybe_str1(Object::ToString(isolate, obj1));
-  MaybeHandle<String> maybe_str2(Object::ToString(isolate, obj2));
-  Handle<String> str1;
-  Handle<String> str2;
-  maybe_str1.ToHandle(&str1);
-  maybe_str2.ToHandle(&str2);
   RETURN_RESULT_OR_FAILURE(isolate,
                            isolate->factory()->NewConsString(str1, str2));
 }
 
 
 RUNTIME_FUNCTION(Runtime_InternalizeString) {
   HandleScope handles(isolate);
   DCHECK_EQ(1, args.length());
   CONVERT_ARG_HANDLE_CHECKED(String, string, 0);
   return *isolate->factory()->InternalizeString(string);
@@ skipping 538 matching lines @@
   SealHandleScope shs(isolate);
   DCHECK_EQ(2, args.length());
   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