We shouldn't throw under FLAG_debug_code, rather abort.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index c2e6ba0fd2e705af50479f7c5f7f1c26e2393db6..4c19fced69dd5b9fa7a20eb4ce28a27db78ddaf4 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -4605,7 +4605,7 @@ void MacroAssembler::EmitSeqStringSetCharCheck(Register string,
                                                uint32_t encoding_mask) {
   Label is_object;
   JumpIfNotSmi(string, &is_object);
-  Throw(kNonObject);
+  Abort(kNonObject);
   bind(&is_object);
 
   push(value);
@@ -4615,17 +4615,17 @@ void MacroAssembler::EmitSeqStringSetCharCheck(Register string,
   andb(value, Immediate(kStringRepresentationMask | kStringEncodingMask));
   cmpq(value, Immediate(encoding_mask));
   pop(value);
-  ThrowIf(not_equal, kUnexpectedStringType);
+  Check(equal, kUnexpectedStringType);
 
   // The index is assumed to be untagged coming in, tag it to compare with the
   // string length without using a temp register, it is restored at the end of
   // this function.
   Integer32ToSmi(index, index);
   SmiCompare(index, FieldOperand(string, String::kLengthOffset));
-  ThrowIf(greater_equal, kIndexIsTooLarge);
+  Check(less, kIndexIsTooLarge);
 
   SmiCompare(index, Smi::FromInt(0));
-  ThrowIf(less, kIndexIsNegative);
+  Check(greater_equal, kIndexIsNegative);
 
   // Restore the index
   SmiToInteger32(index, index);