Keep a trail while checking upper bounds in the VM in order to properly handle

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 85371e08680eb9d48cc397ef9bec78c8e9bf84c7..4a5e36cf69f37ef80896b66f975e1f447ab221d5 100644
--- a/runtime/vm/flow_graph_type_propagator.cc
+++ b/runtime/vm/flow_graph_type_propagator.cc
@@ -412,10 +412,11 @@ void CompileType::Union(CompileType* other) {
 
   const AbstractType* compile_type = ToAbstractType();
   const AbstractType* other_compile_type = other->ToAbstractType();
-  if (compile_type->IsMoreSpecificThan(*other_compile_type, NULL, Heap::kOld)) {
+  if (compile_type->IsMoreSpecificThan(
+          *other_compile_type, NULL, NULL, Heap::kOld)) {
     type_ = other_compile_type;
   } else if (other_compile_type->
-      IsMoreSpecificThan(*compile_type, NULL, Heap::kOld)) {
+      IsMoreSpecificThan(*compile_type, NULL, NULL, Heap::kOld)) {
   // Nothing to do.
   } else {
   // Can't unify.
@@ -618,7 +619,7 @@ bool CompileType::CanComputeIsInstanceOf(const AbstractType& type,
     return false;
   }
 
-  *is_instance = compile_type.IsMoreSpecificThan(type, NULL, Heap::kOld);
+  *is_instance = compile_type.IsMoreSpecificThan(type, NULL, NULL, Heap::kOld);
   return *is_instance;
 }
 
@@ -633,7 +634,7 @@ bool CompileType::IsMoreSpecificThan(const AbstractType& other) {
     return IsNull();
   }
 
-  return ToAbstractType()->IsMoreSpecificThan(other, NULL, Heap::kOld);
+  return ToAbstractType()->IsMoreSpecificThan(other, NULL, NULL, Heap::kOld);
 }