fixes #24711, allow factory constructors to be downcast

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 /// General type checking tests
 library analyzer.test.src.task.strong.checker_test;
 
 import 'package:unittest/unittest.dart';
@@ skipping 1123 matching lines @@
 
   testChecker('super constructor', {
     '/main.dart': '''
           class A { A(A x) {} }
           class B extends A {
             B() : super(/*severe:STATIC_TYPE_ERROR*/3);
           }
        '''
   });
 
+  testChecker('factory constructor downcast', {
+    '/main.dart': r'''
+        class Animal {
+          Animal();
+          factory Animal.cat() => return new Cat();
+        }
+
+        class Cat extends Animal {}
+
+        void main() {
+          Cat c = /*info:ASSIGNMENT_CAST*/new Animal.cat();
+          c = /*severe:STATIC_TYPE_ERROR*/new Animal();
+        }'''
+  });
+
   testChecker('field/field override', {
     '/main.dart': '''
           class A {}
           class B extends A {}
           class C extends B {}
 
           class Base {
             B f1;
             B f2;
             B f3;
@@ skipping 1313 matching lines @@
 
         baz1() sync* { yield* (/*info:DYNAMIC_CAST*/x); }
         Iterable baz2() sync* { yield* (/*info:DYNAMIC_CAST*/x); }
         Iterable<int> baz3() sync* { yield* (/*warning:DOWN_CAST_COMPOSITE*/x); }
         Iterable<int> baz4() sync* { yield* new Iterable<int>(); }
         Iterable<int> baz5() sync* { yield* (/*info:INFERRED_TYPE_ALLOCATION*/new Iterable()); }
     '''
     });
   });
 }