Improve constant pool implementation in the assembler.

runtime/vm/flow_graph.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_FLOW_GRAPH_H_
 #define VM_FLOW_GRAPH_H_
 
 #include "vm/bit_vector.h"
 #include "vm/growable_array.h"
 #include "vm/hash_map.h"
@@ skipping 38 matching lines @@
   typedef ConstantInstr* Pair;
 
   static Key KeyOf(Pair kv) {
     return kv->value();
   }
 
   static Value ValueOf(Pair kv) {
     return kv;
   }
 
+  static Value NoValue() { return NULL; }
+
   static inline intptr_t Hashcode(Key key) {
     if (key.IsSmi()) {
       return Smi::Cast(key).Value();
     }
     if (key.IsDouble()) {
       return static_cast<intptr_t>(
           bit_cast<int32_t, float>(
               static_cast<float>(Double::Cast(key).value())));
     }
     if (key.IsMint()) {
@@ skipping 436 matching lines @@
 
  private:
   GrowableArray<Definition*> defs_;
   BitVector* contains_vector_;
 };
 
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_H_