Adding assertions to fast elements conversion.

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
diff --git a/src/x64/codegen-x64.cc b/src/x64/codegen-x64.cc
index a4880d269b48e1aaea30ec16f493fadef08b5a57..a45592934198a70e974d2c4d274cee9aecc9d28f 100644
--- a/src/x64/codegen-x64.cc
+++ b/src/x64/codegen-x64.cc
@@ -254,6 +254,15 @@ void ElementsTransitionGenerator::GenerateSmiOnlyToDouble(
            xmm0);
   __ jmp(&entry);
   __ bind(&convert_hole);
+
+  if (FLAG_debug_code) {
+    Label is_really_the_hole;
+    __ CompareRoot(rbx, Heap::kTheHoleValueRootIndex);
+    __ j(equal, &is_really_the_hole);
+    __ Abort("object found in smi-only array");
+    __ bind(&is_really_the_hole);
+  }
+
   __ movq(FieldOperand(r14, r9, times_8, FixedDoubleArray::kHeaderSize), r15);
   __ bind(&entry);
   __ decq(r9);