Access ConstantSystem through ClosedWorld.

pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index d3fe8de13bb02231575f9a10bb1010dd1fae609f..332e256ac1617105bce0c7a84c9ea5b13b2771cf 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -863,8 +863,7 @@ class TypeGraphInferrerEngine
                   // Although we might find a better type, we have to keep
                   // the old type around to ensure that we get a complete view
                   // of the type graph and do not drop any flow edges.
-                  TypeMask refinedType =
-                      computeTypeMask(closedWorld, compiler.backend, value);
+                  TypeMask refinedType = computeTypeMask(closedWorld, value);
                   assert(TypeMask.assertIsNormalized(refinedType, closedWorld));
                   type = new NarrowTypeInformation(type, refinedType);
                   types.allocatedTypes.add(type);