fixes #24711, allow factory constructors to be downcast

pkg/analyzer/lib/src/task/strong/info.dart

Index: pkg/analyzer/lib/src/task/strong/info.dart
diff --git a/pkg/analyzer/lib/src/task/strong/info.dart b/pkg/analyzer/lib/src/task/strong/info.dart
index cf4f8226714d4f5000d77144aabbec8cde263ec4..b6882bc4368293ddd41074a32329e6acc3b5f637 100644
--- a/pkg/analyzer/lib/src/task/strong/info.dart
+++ b/pkg/analyzer/lib/src/task/strong/info.dart
@@ -133,9 +133,12 @@ abstract class DownCast extends CoercionInfo {
       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