Add a test for reporting a duplicate constructor.

tests/language/src/DuplicateConstructorTest.dart

Index: tests/language/src/DuplicateConstructorTest.dart
===================================================================
--- tests/language/src/DuplicateConstructorTest.dart	(revision 0)
+++ tests/language/src/DuplicateConstructorTest.dart	(revision 0)
@@ -0,0 +1,12 @@
+// Copyright (c) 2011, 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.
+
+class Foo {
+  Foo();
+  Foo(); /// 01: compile-time error
+}
+
+main() {
+  new Foo();
+}