Do not use guarded cid on externalizable cid-s (one and two byte strings).

runtime/vm/flow_graph_type_propagator.cc

Index: runtime/vm/flow_graph_type_propagator.cc
===================================================================
--- runtime/vm/flow_graph_type_propagator.cc	(revision 25890)
+++ runtime/vm/flow_graph_type_propagator.cc	(working copy)
@@ -945,14 +945,19 @@
   }
 
   if (FLAG_enable_type_checks) {
+    ASSERT(!type().HasResolvedTypeClass() ||
+           !Field::IsExternalizableCid(Class::Handle(
+                type().type_class()).id()));
     return CompileType::FromAbstractType(type());
   }
 
-  if (field_ != NULL && (field_->guarded_cid() != kIllegalCid)) {
+  if ((field_ != NULL) && (field_->guarded_cid() != kIllegalCid)) {
+    ASSERT(!Field::IsExternalizableCid(field_->guarded_cid()));
     return CompileType::CreateNullable(field_->is_nullable(),
                                        field_->guarded_cid());
   }
 
+  ASSERT(!Field::IsExternalizableCid(result_cid_));
   return CompileType::FromCid(result_cid_);
 }