Store named parameters in sorted lists instead of using maps.

lib/transformations/treeshaker.dart

Index: lib/transformations/treeshaker.dart
diff --git a/lib/transformations/treeshaker.dart b/lib/transformations/treeshaker.dart
index 4eb69073b9560f42e824577de7a14eccb6029200..bd33595d2e8885218cb405893a1f59983890d801 100644
--- a/lib/transformations/treeshaker.dart
+++ b/lib/transformations/treeshaker.dart
@@ -757,7 +757,9 @@ class _ExternalTypeVisitor extends DartTypeVisitor {
     for (int i = 0; i < node.positionalParameters.length; ++i) {
       visitContravariant(node.positionalParameters[i]);
     }
-    node.namedParameters.values.forEach(visitContravariant);
+    for (int i = 0; i < node.namedParameters.length; ++i) {
+      visitContravariant(node.namedParameters[i].type);
+    }
   }
 
   visitTypeParameterType(TypeParameterType node) {}