Fix issue 16375: disallow marking null-constant types as non-nullable. The issue occurs for strict …

runtime/vm/flow_graph_type_propagator.cc

Index: runtime/vm/flow_graph_type_propagator.cc
===================================================================
--- runtime/vm/flow_graph_type_propagator.cc	(revision 32222)
+++ runtime/vm/flow_graph_type_propagator.cc	(working copy)
@@ -96,7 +96,9 @@
         BranchInstr* branch = instr->AsBranch();
         if (branch != NULL) {
           ConstrainedCompileType* constrained_type = branch->constrained_type();
-          if (constrained_type != NULL) constrained_type->Update();
+          if (constrained_type != NULL) {
+            constrained_type->Update();
+          }
         }
       }
     }
@@ -225,7 +227,7 @@
 ConstrainedCompileType* FlowGraphTypePropagator::MarkNonNullable(
     Definition* def) {
   CompileType* current = TypeOf(def);
-  if (current->is_nullable()) {
+  if (current->is_nullable() && (current->ToCid() != kNullCid)) {
     ConstrainedCompileType* constrained_type =
         new NotNullConstrainedCompileType(current);
     SetTypeOf(def, constrained_type->ToCompileType());