Fold away x === null comparisons when propagated cid of x is not kDynamicCid.

runtime/vm/intermediate_language.h

 // Copyright (c) 2012, 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 1159 matching lines @@
   virtual bool CanDeoptimize() const { return false; }
 
   virtual bool HasSideEffect() const { return false; }
 
   // TODO(regis): This helper will be removed once we support type sets.
   RawAbstractType* LeastSpecificInputType() const;
 
   // Phi is alive if it reaches a non-environment use.
   bool is_alive() const { return is_alive_; }
   void mark_alive() { is_alive_ = true; }
+  void mark_dead() { is_alive_ = false; }
 
   virtual Representation RequiredInputRepresentation(intptr_t i) const {
     return representation_;
   }
 
   virtual Representation representation() const {
     return representation_;
   }
 
   virtual void set_representation(Representation r) {
@@ skipping 2981 matching lines @@
   ForwardInstructionIterator* current_iterator_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FlowGraphVisitor);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_INTERMEDIATE_LANGUAGE_H_