Add constructor test

tests/language/constructor_initializer_test.dart

+// Copyright (c) 2013, 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 A {
+  var _x, _y;
+  A(x, [y = 10]): _x = x++, _y = y++ {
+    // Check that value of modified constructor parameters
+    // is remembered in the constructor body.
+    Expect.equals(x, _x + 1);
+    Expect.equals(y, _y + 1);
+    Expect.isFalse(?y);
+  }
+}
+
+class B extends A {
+  var _a, _b;
+  // The super call in the middle of the initializer list conceptually
+  // forces two super call chains, one for initializer list and a second
+  // one for the constructor bodies.
+  B(a, b): _a = a++, super(a + b++), _b = b++ {
+    Expect.equals(a, _a + 1);
+    Expect.equals(b, _b + 1);
+    Expect.equals(a + (b-2), _x);
+  }
+}
+
+main() {
+  var o = new B(3, 5);
+  Expect.equals(3, o._a);
+  Expect.equals(6, o._b);
+  Expect.equals(9, o._x);
+  Expect.equals(10, o._y);
+  o = new A(3);
+  Expect.equals(3, o._x);
+  Expect.equals(10, o._y);
+}