Fix VM crash when parsing a non-existing redirecting constructor

tests/language/regress_28217_test.dart

+// Copyright (c) 2017, 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.
+
+// Test non-existing redirecting constructor and
+// redirecting to a factory constructor.
+
+class B {
+  B() : this.a();    /// none: compile-time error
+  factory B.a() { }  /// 01: compile-time error
+  B.a();             /// 02: ok
+}
+
+main() => new B();
+