Add compiler error when instantiating abstract class

tests/language/implicit_this_test.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.
 
 interface Interface {
   final x;
 }
 
 // The abstract keyword is implicit if a class declares an abstract
 // member. This class does not declare any abstract members, so it
@@ skipping 19 matching lines @@
 class Concrete extends Abstract {
   get x => 7;
 }
 
 class SubConcrete extends Concrete {
   final x;
   SubConcrete(this.x);
 }
 
 void main() {
-  var x = new Abstract(); /// 02: static type warning
+  var x = new Abstract(); /// 02: compile-time error
   var y = new SubAbstract1(); /// 01: continued
-  var z = new SubAbstract2(); /// 03: static type warning
+  var z = new SubAbstract2(); /// 03: compile-time error
   var a = new SubSubAbstract2(); /// 04: continued
   Expect.equals(x, x); /// 02: continued
   Expect.equals('7', new Concrete().toString());
   Expect.equals('42', new SubConcrete(42).toString());
   Expect.equals('7', new SubConcrete(new Concrete()).toString());
 }