add a regression test for #26431, which is now fixed

pkg/analyzer/test/src/task/strong/inferred_type_test.dart

Index: pkg/analyzer/test/src/task/strong/inferred_type_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
index 32000e99686c3ced31ac7b2e5660275db7477a37..fe2bdfb5b3bf259875a65f89eba2c612f3d953e7 100644
--- a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
+++ b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
@@ -588,6 +588,16 @@ class C2 implements A, B {
 ''');
   }
 
+  void test_constructors_downwardsWithConstraint() {
+    // Regression test for https://github.com/dart-lang/sdk/issues/26431
+    checkFile(r'''
+class Foo<T extends Iterable> {}
+void main() {
+  Foo<List> foo = /*info:INFERRED_TYPE_ALLOCATION*/new Foo();
+}
+    ''');
+  }
+
   void test_constructors_inferenceFBounded() {
     // Regression for https://github.com/dart-lang/sdk/issues/26990
     var unit = checkFile('''