Inline instance object hash code into object header on 64 bit.

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index 64b7499d7962a25f0b85cf721737264c3ca1aa5c..4f88d2202ad5e36e753de689b87e6e61de0afca7 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -33,7 +33,11 @@ class Heap {
     kCode,
   };
 
+#if defined(HASH_IN_OBJECT_HEADER)
+  enum WeakSelector { kPeers = 0, kObjectIds, kNumWeakSelectors };
+#else
   enum WeakSelector { kPeers = 0, kHashes, kObjectIds, kNumWeakSelectors };
+#endif
 
   enum ApiCallbacks { kIgnoreApiCallbacks, kInvokeApiCallbacks };
 
@@ -167,6 +171,7 @@ class Heap {
   }
   int64_t PeerCount() const;
 
+#if !defined(HASH_IN_OBJECT_HEADER)
   // Associate an identity hashCode with an object. An non-existent hashCode
   // is equal to 0.
   void SetHash(RawObject* raw_obj, intptr_t hash) {
@@ -175,6 +180,7 @@ class Heap {
   intptr_t GetHash(RawObject* raw_obj) const {
     return GetWeakEntry(raw_obj, kHashes);
   }
+#endif
   int64_t HashCount() const;
 
   // Associate an id with an object (used when serializing an object).