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

runtime/vm/intermediate_language_mips.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_MIPS.
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 4652 matching lines @@
   }
   return summary;
 }
 
 
 void CheckClassInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   Label* deopt = compiler->AddDeoptStub(deopt_id(),
                                         ICData::kDeoptCheckClass,
                                         licm_hoisted_ ? ICData::kHoisted : 0);
   if (IsNullCheck()) {
-    __ BranchEqual(locs()->in(0).reg(), Object::null_object(), deopt);
+    if (DeoptIfNull()) {
+      __ BranchEqual(locs()->in(0).reg(), Object::null_object(), deopt);
+    } else {
+      ASSERT(DeoptIfNotNull());
+      __ BranchNotEqual(locs()->in(0).reg(), Object::null_object(), 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) {
     __ andi(CMPRES1, value, Immediate(kSmiTagMask));
@@ skipping 936 matching lines @@
                                 1,
                                 locs());
   __ lw(result, Address(SP, 1 * kWordSize));
   __ addiu(SP, SP, Immediate(2 * kWordSize));
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS