Add compiler error when instantiating abstract class

tests/language/interface_test.dart

-// Copyright (c) 2012, 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 testing Interfaces.
-
-interface Ai {
-  int foo();
-}
-
-interface Bi extends Ai default InterfaceTest {
-  Bi();
-}
-
-interface Simple extends Ai { }
-
-interface Aai { }
-
-interface Abi { }
-
-interface Bar { }
-
-interface Foo extends Bar { }
-
-interface Baz extends Bar, Foo { }
-
-class InterfaceTest implements Ai, Aai, Abi, Baz, Bi {
-  var f;
-
-  InterfaceTest() {}
-  int foo() { return 1; }
-
-  // intentionally unimplemented methods
-  abstract beta();
-  abstract String beta1();
-  abstract String beta2(double d);
-}
-
-main() {
-  // instantiate an abstract class
-  var o = new Bi(); /// static type warning
-  Expect.equals(1, o.foo());
-}