Properly handle forwarding factory constructors in type inference.

tests/language/forwarding_factory_constructor_default_values_test.dart

+// Copyright (c) 2014, 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.
+
+// Regression test for dart2js bug 18257: Properly infer types for forwarding
+// factory constructors with optional parameters with default values.
+
+main() {
+  A a = new A.a1();
+  a.test();
+}
+
+class A {
+  final bool condition;
+
+  A({this.condition: true});
+
+  factory A.a1({condition}) = _A1.boo;
+
+  test() {
+    if(condition != true) {
+      throw "FAILED";
+    } 
+  }
+}
+
+class _A1 extends A {
+  _A1.boo({condition: true}):
+    super(condition: condition);
+}
+