Improve constant pool implementation in the assembler.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 42801)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -1023,44 +1023,6 @@
   // Patchability of pool entries.
   GrowableArray<Patchability> patchable_pool_entries_;
 
-  // Pair type parameter for DirectChainedHashMap.
-  class ObjIndexPair {
-   public:
-    // TODO(zra): A WeakTable should be used here instead, but then it would
-    // also have to be possible to register and de-register WeakTables with the
-    // heap. Also, the Assembler would need to become a StackResource.
-    // Issue 13305. In the meantime...
-    // CAUTION: the RawObject* below is only safe because:
-    // The HashMap that will use this pair type will not contain any RawObject*
-    // keys that are not in the object_pool_ array. Since the keys will be
-    // visited by the GC when it visits the object_pool_, and since all objects
-    // in the object_pool_ are Old (and so will not be moved) the GC does not
-    // also need to visit the keys here in the HashMap.
-
-    // Typedefs needed for the DirectChainedHashMap template.
-    typedef RawObject* Key;
-    typedef intptr_t Value;
-    typedef ObjIndexPair Pair;
-
-    ObjIndexPair(Key key, Value value) : key_(key), value_(value) { }
-
-    static Key KeyOf(Pair kv) { return kv.key_; }
-
-    static Value ValueOf(Pair kv) { return kv.value_; }
-
-    static intptr_t Hashcode(Key key) {
-      return reinterpret_cast<intptr_t>(key) >> kObjectAlignmentLog2;
-    }
-
-    static inline bool IsKeyEqual(Pair kv, Key key) {
-      return kv.key_ == key;
-    }
-
-   private:
-    Key key_;
-    Value value_;
-  };
-
   // Hashmap for fast lookup in object pool.
   DirectChainedHashMap<ObjIndexPair> object_pool_index_table_;