Adds an assertion to limit snapshot intptr_t size.

runtime/vm/raw_object_snapshot.cc

Index: runtime/vm/raw_object_snapshot.cc
===================================================================
--- runtime/vm/raw_object_snapshot.cc	(revision 37116)
+++ runtime/vm/raw_object_snapshot.cc	(working copy)
@@ -2381,13 +2381,13 @@
   ASSERT(kind != Snapshot::kFull);
   intptr_t cid = RawObject::ClassIdTag::decode(tags);
   intptr_t length = reader->ReadSmiValue();
-  uint8_t* data = reinterpret_cast<uint8_t*>(reader->ReadIntptrValue());
+  uint8_t* data = reinterpret_cast<uint8_t*>(reader->ReadRawPointerValue());
   const ExternalTypedData& obj = ExternalTypedData::Handle(
       ExternalTypedData::New(cid, data, length));
-  void* peer = reinterpret_cast<void*>(reader->ReadIntptrValue());
+  void* peer = reinterpret_cast<void*>(reader->ReadRawPointerValue());
   Dart_WeakPersistentHandleFinalizer callback =
       reinterpret_cast<Dart_WeakPersistentHandleFinalizer>(
-          reader->ReadIntptrValue());
+          reader->ReadRawPointerValue());
   obj.AddFinalizer(peer, callback);
   return obj.raw();
 }