We shouldn't throw under FLAG_debug_code, rather abort.

src/x64/full-codegen-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 3374 matching lines @@
   Register index = rbx;
   Register value = rcx;
 
   VisitForStackValue(args->at(1));  // index
   VisitForStackValue(args->at(2));  // value
   VisitForAccumulatorValue(args->at(0));  // string
   __ pop(value);
   __ pop(index);
 
   if (FLAG_debug_code) {
-    __ ThrowIf(NegateCondition(__ CheckSmi(value)), kNonSmiValue);
-    __ ThrowIf(NegateCondition(__ CheckSmi(index)), kNonSmiValue);
+    __ Check(__ CheckSmi(value), kNonSmiValue);
+    __ Check(__ CheckSmi(index), kNonSmiValue);
   }
 
   __ SmiToInteger32(value, value);
   __ SmiToInteger32(index, index);
 
   if (FLAG_debug_code) {
     static const uint32_t one_byte_seq_type = kSeqStringTag | kOneByteStringTag;
     __ EmitSeqStringSetCharCheck(string, index, value, one_byte_seq_type);
   }
 
@@ skipping 11 matching lines @@
   Register index = rbx;
   Register value = rcx;
 
   VisitForStackValue(args->at(1));  // index
   VisitForStackValue(args->at(2));  // value
   VisitForAccumulatorValue(args->at(0));  // string
   __ pop(value);
   __ pop(index);
 
   if (FLAG_debug_code) {
-    __ ThrowIf(NegateCondition(__ CheckSmi(value)), kNonSmiValue);
-    __ ThrowIf(NegateCondition(__ CheckSmi(index)), kNonSmiValue);
+    __ Check(__ CheckSmi(value), kNonSmiValue);
+    __ Check(__ CheckSmi(index), kNonSmiValue);
   }
 
   __ SmiToInteger32(value, value);
   __ SmiToInteger32(index, index);
 
   if (FLAG_debug_code) {
     static const uint32_t two_byte_seq_type = kSeqStringTag | kTwoByteStringTag;
     __ EmitSeqStringSetCharCheck(string, index, value, two_byte_seq_type);
   }
 
@@ skipping 1453 matching lines @@
 
   ASSERT_EQ(isolate->builtins()->OsrAfterStackCheck()->entry(),
             Assembler::target_address_at(call_target_address));
   return OSR_AFTER_STACK_CHECK;
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64