fixes #24711, allow factory constructors to be downcast

pkg/analyzer/lib/src/task/strong/info.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.
 
 /// Defines static information collected by the type checker and used later by
 /// emitters to generate code.
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.info;
 
@@ skipping 115 matching lines @@
       // fromT should be an exact type - this will almost certainly fail at
       // runtime.
       return new StaticTypeError(rules, expression, toT, reason: reason);
     }
     if (expression is FunctionExpression) {
       // fromT should be an exact type - this will almost certainly fail at
       // runtime.
       return new UninferredClosure(rules, expression, cast);
     }
     if (expression is InstanceCreationExpression) {
-      // fromT should be an exact type - this will almost certainly fail at
-      // runtime.
-      return new StaticTypeError(rules, expression, toT, reason: reason);
+      ConstructorElement e = expression.staticElement;
+      if (e == null || !e.isFactory) {
+        // fromT should be an exact type - this will almost certainly fail at
+        // runtime.
+        return new StaticTypeError(rules, expression, toT, reason: reason);
+      }
     }
 
     // TODO(vsm): Change this to an assert when we have generic methods and
     // fix TypeRules._coerceTo to disallow implicit sideways casts.
     if (!rules.isSubtypeOf(toT, fromT)) {
       assert(toT.isSubtypeOf(fromT) || fromT.isAssignableTo(toT));
       return new DownCastComposite(rules, expression, cast);
     }
 
     // Composite cast: these are more likely to fail.
@@ skipping 413 matching lines @@
 // TODO(vsm,leafp): Remove this.
 class UninferredClosure extends DownCast {
   UninferredClosure(TypeSystem rules, FunctionExpression expression, Cast cast)
       : super._internal(rules, expression, cast);
 
   @override
   String get name => 'STRONG_MODE_UNINFERRED_CLOSURE';
 
   toErrorCode() => new StaticTypeWarningCode(name, message);
 }