Use ICU case conversion/transliterator for case conversion behind a flag

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/regexp/jsregexp-inl.h"
 #include "src/string-builder.h"
 #include "src/string-search.h"
@@ skipping 1059 matching lines @@
     if (length < 0) length = -length;
     ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
         isolate, result, isolate->factory()->NewRawTwoByteString(length));
   }
   return ConvertCaseHelper(isolate, *s, *result, length, mapping);
 }
 
 
 RUNTIME_FUNCTION(Runtime_StringToLowerCase) {
   HandleScope scope(isolate);
-  DCHECK(args.length() == 1);
+  DCHECK_EQ(args.length(), 1);
   CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
   return ConvertCase(s, isolate, isolate->runtime_state()->to_lower_mapping());
 }
 
 
 RUNTIME_FUNCTION(Runtime_StringToUpperCase) {
   HandleScope scope(isolate);
-  DCHECK(args.length() == 1);
+  DCHECK_EQ(args.length(), 1);
   CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
   return ConvertCase(s, isolate, isolate->runtime_state()->to_upper_mapping());
 }
 
 
 RUNTIME_FUNCTION(Runtime_StringTrim) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 3);
 
   CONVERT_ARG_HANDLE_CHECKED(String, string, 0);
@@ skipping 223 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