Enable downward inference on default params

test/checker/checker_test.dart

Index: test/checker/checker_test.dart
diff --git a/test/checker/checker_test.dart b/test/checker/checker_test.dart
index f35990c3fcd9ad5c9fd696d381006b91ff3e6f01..b499d4a9bb257819723c0528099e843e0ff70d3e 100644
--- a/test/checker/checker_test.dart
+++ b/test/checker/checker_test.dart
@@ -167,7 +167,7 @@ void main() {
             T /*severe:InvalidVariableDeclaration*/z;
           }
 
-          void baz(T x, [T /*severe:InvalidVariableDeclaration*/y, T z = /*severe:StaticTypeError*/null]) {
+          void baz(T x, [T /*severe:InvalidVariableDeclaration*/y, T z = /*warning:DownCastImplicit*/null]) {
           }
         }