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

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 44874)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -191,7 +191,7 @@
 }
 
 
-bool CheckClassInstr::IsNullCheck() const {
+bool CheckClassInstr::DeoptIfNull() const {
   if (unary_checks().NumberOfChecks() != 1) {
     return false;
   }
@@ -203,6 +203,19 @@
 }
 
 
+// Null object is a singleton of null-class (except for some sentinel,
+// transitional temporaries). Instead of checking against the null class only
+// we can check against null instance instead.
+bool CheckClassInstr::DeoptIfNotNull() const {
+  if (unary_checks().NumberOfChecks() != 1) {
+    return false;
+  }
+  const intptr_t cid = unary_checks().GetCidAt(0);
+  return cid == kNullCid;
+}
+
+
+
 bool CheckClassInstr::IsDenseSwitch() const {
   if (unary_checks().GetReceiverClassIdAt(0) == kSmiCid) return false;
   if (cids_.length() > 2 &&