Add test that checks that the default constructor is called.

tests/language/src/ConstructorDefaultTest.dart

+// 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.
+// Dart test program for default constructors.
+
+class A {
+  A() : a = 499;
+
+  var a;
+}
+
+class B extends A {
+  B() { Expect.equals(499, a); }
+}
+
+main() {
+  new B();
+}