A CheckClass instruction that compares against a null class only can be optimized to check instance…

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
===================================================================
--- runtime/vm/intermediate_language_x64.cc	(revision 44874)
+++ runtime/vm/intermediate_language_x64.cc	(working copy)
@@ -5503,7 +5503,8 @@
   if (IsNullCheck()) {
     __ CompareObject(locs()->in(0).reg(),
                      Object::null_object(), PP);
-    __ j(EQUAL, deopt);
+    Condition cond = DeoptIfNull() ? EQUAL : NOT_EQUAL;
+    __ j(cond, deopt);
     return;
   }