Ensure that type of the AssertAssignable instruction is recomputed.

runtime/vm/flow_graph_type_propagator.cc

Index: runtime/vm/flow_graph_type_propagator.cc
diff --git a/runtime/vm/flow_graph_type_propagator.cc b/runtime/vm/flow_graph_type_propagator.cc
index ec1a880c9c464201b8496429ffa9a34930731961..827f9e47b194bc5f01b4835f50c9823b4ccfd580 100644
--- a/runtime/vm/flow_graph_type_propagator.cc
+++ b/runtime/vm/flow_graph_type_propagator.cc
@@ -734,11 +734,9 @@ bool AssertAssignableInstr::RecomputeType() {
     return false;
   }
 
-  if (value_type->IsMoreSpecificThan(dst_type())) {
-    return UpdateType(*value_type);
-  }
-
-  return false;
+  return UpdateType(value_type->IsMoreSpecificThan(dst_type())
+      ? *value_type
+      : CompileType::FromAbstractType(dst_type()));
 }