ReceiverCheckMode needs to be utilized further.

src/arm64/code-stubs-arm64.cc

 // Copyright 2013 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/v8.h"
 
 #if V8_TARGET_ARCH_ARM64
 
 #include "src/bootstrapper.h"
 #include "src/code-stubs.h"
@@ skipping 3794 matching lines @@
 
   __ Bind(&runtime);
   __ TailCallRuntime(Runtime::kSubString, 3, 1);
 
   __ bind(&single_char);
   // x1: result_length
   // x10: input_string
   // x12: input_type
   // x15: from (untagged)
   __ SmiTag(from);
-  StringCharAtGenerator generator(
-      input_string, from, result_length, x0,
-      &runtime, &runtime, &runtime, STRING_INDEX_IS_NUMBER);
+  StringCharAtGenerator generator(input_string, from, result_length, x0,
+                                  &runtime, &runtime, &runtime,
+                                  STRING_INDEX_IS_NUMBER, RECEIVER_IS_STRING);
   generator.GenerateFast(masm);
   __ Drop(3);
   __ Ret();
   generator.SkipSlow(masm, &runtime);
 }
 
 
 void StringHelper::GenerateFlatOneByteStringEquals(
     MacroAssembler* masm, Register left, Register right, Register scratch1,
     Register scratch2, Register scratch3) {
@@ skipping 1262 matching lines @@
                               MemOperand(fp, 6 * kPointerSize),
                               NULL);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM64