Implement IsNullCheck for CheckClassInstr. If input is nullable expected type, then check for null …

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 25902)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -86,8 +86,7 @@
 CheckClassInstr::CheckClassInstr(Value* value,
                                  intptr_t deopt_id,
                                  const ICData& unary_checks)
-    : unary_checks_(unary_checks),
-      null_check_(false) {
+    : unary_checks_(unary_checks) {
   ASSERT(unary_checks.IsZoneHandle());
   // Expected useful check data.
   ASSERT(!unary_checks_.IsNull());
@@ -128,6 +127,16 @@
 }
 
 
+bool CheckClassInstr::IsNullCheck() const {
+  if (unary_checks().NumberOfChecks() != 1) {
+    return false;
+  }
+  CompileType* in_type = value()->Type();
+  const intptr_t cid = unary_checks().GetCidAt(0);
+  return in_type->is_nullable() && (in_type->ToNullableCid() == cid);
+}
+
+
 bool GuardFieldInstr::AttributesEqual(Instruction* other) const {
   return field().raw() == other->AsGuardField()->field().raw();
 }