Version 3.5.5.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
===================================================================
--- src/x64/full-codegen-x64.cc	(revision 8931)
+++ src/x64/full-codegen-x64.cc	(working copy)
@@ -3971,6 +3971,10 @@
     __ j(equal, if_true);
     __ CompareRoot(rax, Heap::kFalseValueRootIndex);
     Split(equal, if_true, if_false, fall_through);
+  } else if (FLAG_harmony_typeof &&
+             check->Equals(isolate()->heap()->null_symbol())) {
+    __ CompareRoot(rax, Heap::kNullValueRootIndex);
+    Split(equal, if_true, if_false, fall_through);
   } else if (check->Equals(isolate()->heap()->undefined_symbol())) {
     __ CompareRoot(rax, Heap::kUndefinedValueRootIndex);
     __ j(equal, if_true);
@@ -3987,8 +3991,10 @@
     Split(above_equal, if_true, if_false, fall_through);
   } else if (check->Equals(isolate()->heap()->object_symbol())) {
     __ JumpIfSmi(rax, if_false);
-    __ CompareRoot(rax, Heap::kNullValueRootIndex);
-    __ j(equal, if_true);
+    if (!FLAG_harmony_typeof) {
+      __ CompareRoot(rax, Heap::kNullValueRootIndex);
+      __ j(equal, if_true);
+    }
     __ CmpObjectType(rax, FIRST_NONCALLABLE_SPEC_OBJECT_TYPE, rdx);
     __ j(below, if_false);
     __ CmpInstanceType(rdx, LAST_NONCALLABLE_SPEC_OBJECT_TYPE);