A test to see that the default clause of a interface can use an undeclared default constructor

tests/language/src/DefaultClassImplicitConstructorTest.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.
+
+// An interface with a default class with no declared constructor should 
+// use the implicit default constructor
+
+interface A default B {
+  A();
+}
+
+class B implements A {
+}
+
+main() {
+  var val = new A();
+  Expect.equals(true, val is A);
+  Expect.equals(true, val is B);
+}