Fix crash when reading the Richards snapshot generated on using a the 32 bit dart binary.

runtime/vm/raw_object_snapshot.cc

Index: runtime/vm/raw_object_snapshot.cc
diff --git a/runtime/vm/raw_object_snapshot.cc b/runtime/vm/raw_object_snapshot.cc
index 26bac91ad44d6a76e46074eff7f160dd6db64581..85c5d748af480a460a492393a78e32b5f4878448 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -1830,7 +1830,10 @@ RawInteger* Mint::ReadFrom(SnapshotReader* reader,
   // Check if the value could potentially fit in a Smi in our current
   // architecture, if so return the object as a Smi.
   if (Smi::IsValid(value)) {
-    return Smi::New(static_cast<intptr_t>(value));
+    Smi& smi = Smi::ZoneHandle(reader->zone(),
+                               Smi::New(static_cast<intptr_t>(value)));
+    reader->AddBackRef(object_id, &smi, kIsDeserialized);
+    return smi.raw();
   }
 
   // Create a Mint object or get canonical one if it is a canonical constant.