Become: Also forward identity hashes.

runtime/vm/become.cc

Index: runtime/vm/become.cc
diff --git a/runtime/vm/become.cc b/runtime/vm/become.cc
index 0850584e86ce0431ef41917da73ad5bf34bf9d45..5424805bb65ec83d4f20d46eff2529edfe15127d 100644
--- a/runtime/vm/become.cc
+++ b/runtime/vm/become.cc
@@ -261,6 +261,13 @@ void Become::ElementsForwardIdentity(const Array& before, const Array& after) {
     }
 
     ForwardObjectTo(before_obj, after_obj);
+
+    // Forward the identity hash too if it has one.
+    intptr_t hash = heap->GetHash(before_obj);
+    if (hash != 0) {
+      ASSERT(heap->GetHash(after_obj) == 0);
+      heap->SetHash(after_obj, hash);
+    }

[siva, 2016/08/19 00:08:15]

How does this work for internal VM objects, for example if before_obj and after
_obj are strings then it is not correct to get the hash of before_obj and set it
to the hash of after_obj?

[rmacnak, 2016/08/19 00:16:54]

A String's usual hash is the one inline in the object. This forwards identity
hashes in the weak tables.

   }
 
   {