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

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 488739cfbf2f115848aa9a7b095500405043f7be..33b9e784f0be3a045324226648126b2bc39a0be8 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -271,6 +271,10 @@ class RawObject {
     kSizeTagSize = 8,
     kClassIdTagPos = kSizeTagPos + kSizeTagSize,  // = 16
     kClassIdTagSize = 16,
+#if defined(HASH_IN_OBJECT_HEADER)
+    kHashTagPos = kClassIdTagPos + kClassIdTagSize,  // = 32
+    kHashTagSize = 16,
+#endif
   };
 
   COMPILE_ASSERT(kClassIdTagSize == (sizeof(classid_t) * kBitsPerByte));
@@ -409,6 +413,7 @@ class RawObject {
 #undef DEFINE_IS_CID
 
   bool IsStringInstance() const { return IsStringClassId(GetClassId()); }
+  bool IsRawNull() const { return GetClassId() == kNullCid; }
   bool IsDartInstance() const {
     return (!IsHeapObject() || (GetClassId() >= kInstanceCid));
   }