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

runtime/vm/intermediate_language_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 5485 matching lines @@
 }
 
 
 void CheckClassInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   Label* deopt = compiler->AddDeoptStub(deopt_id(),
                                         ICData::kDeoptCheckClass,
                                         licm_hoisted_ ? ICData::kHoisted : 0);
   if (IsNullCheck()) {
     __ CompareObject(locs()->in(0).reg(),
                      Object::null_object(), PP);
-    __ j(EQUAL, deopt);
+    Condition cond = DeoptIfNull() ? EQUAL : NOT_EQUAL;
+    __ j(cond, deopt);
     return;
   }
 
   ASSERT((unary_checks().GetReceiverClassIdAt(0) != kSmiCid) ||
          (unary_checks().NumberOfChecks() > 1));
   Register value = locs()->in(0).reg();
   Register temp = locs()->temp(0).reg();
   Label is_ok;
   if (unary_checks().GetReceiverClassIdAt(0) == kSmiCid) {
     __ testq(value, Immediate(kSmiTagMask));
@@ skipping 925 matching lines @@
   __ Drop(1);
   __ popq(result);
 }
 
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64