Throw AbstractClassInstantiationError if an abstract class is instantiated

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 {
@@ skipping 19 matching lines @@
   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());
+  var o = new Bi(); /// 00: static type warning, runtime error
 }