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

runtime/vm/intermediate_language.h

 // 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.
 
 #ifndef VM_INTERMEDIATE_LANGUAGE_H_
 #define VM_INTERMEDIATE_LANGUAGE_H_
 
 #include "vm/allocation.h"
 #include "vm/ast.h"
 #include "vm/growable_array.h"
@@ skipping 7574 matching lines @@
   Value* value() const { return inputs_[0]; }
 
   const ICData& unary_checks() const { return unary_checks_; }
 
   const GrowableArray<intptr_t>& cids() const { return cids_; }
 
   virtual Instruction* Canonicalize(FlowGraph* flow_graph);
 
   virtual void PrintOperandsTo(BufferFormatter* f) const;
 
-  bool IsNullCheck() const;
+  bool IsNullCheck() const {
+    return  DeoptIfNull() || DeoptIfNotNull();
+  }
+
+  bool DeoptIfNull() const;
+  bool DeoptIfNotNull() const;
 
   bool IsDenseSwitch() const;
   intptr_t ComputeCidMask() const;
   static bool IsDenseMask(intptr_t mask);
 
   virtual bool AllowsCSE() const { return true; }
   virtual EffectSet Dependencies() const;
   virtual EffectSet Effects() const { return EffectSet::None(); }
   virtual bool AttributesEqual(Instruction* other) const;
 
@@ skipping 462 matching lines @@
   LocationSummary* Name::MakeLocationSummary(Zone* zone, bool opt) const {    \
     UNIMPLEMENTED();                                                          \
     return NULL;                                                              \
   }                                                                           \
   void Name::EmitNativeCode(FlowGraphCompiler* compiler) { UNIMPLEMENTED(); }
 
 
 }  // namespace dart
 
 #endif  // VM_INTERMEDIATE_LANGUAGE_H_