[builtins] migrate C++ String Iterator builtins to baseline TurboFan

src/code-stub-assembler.h

 // Copyright 2016 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.
 
 #ifndef V8_CODE_STUB_ASSEMBLER_H_
 #define V8_CODE_STUB_ASSEMBLER_H_
 
 #include <functional>
 
 #include "src/compiler/code-assembler.h"
@@ skipping 426 matching lines @@
                               PrimitiveType primitive_type,
                               char const* method_name);
 
   // String helpers.
   // Load a character from a String (might flatten a ConsString).
   compiler::Node* StringCharCodeAt(compiler::Node* string,
                                    compiler::Node* smi_index);
   // Return the single character string with only {code}.
   compiler::Node* StringFromCharCode(compiler::Node* code);
 
+  // Return a String made up of the codepoint at index.
+  compiler::Node* StringFromCodePointAt(compiler::Node* string,
+                                        compiler::Node* length,
+                                        compiler::Node* index);
+
   // Type conversion helpers.
   // Convert a String to a Number.
   compiler::Node* StringToNumber(compiler::Node* context,
                                  compiler::Node* input);
   // Convert an object to a name.
   compiler::Node* ToName(compiler::Node* context, compiler::Node* input);
   // Convert a Non-Number object to a Number.
   compiler::Node* NonNumberToNumber(compiler::Node* context,
                                     compiler::Node* input);
   // Convert any object to a Number.
@@ skipping 340 matching lines @@
                                            Label* bailout);
 
   static const int kElementLoopUnrollThreshold = 8;
 };
 
 DEFINE_OPERATORS_FOR_FLAGS(CodeStubAssembler::AllocationFlags);
 
 }  // namespace internal
 }  // namespace v8
 #endif  // V8_CODE_STUB_ASSEMBLER_H_