Eliminate explicit abstract from class members in tests

tests/language/typed_selector_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.
 
 // Regression test for http://dartbug.com/6259. This test used to fail
 // on dart2js because class A does not know [A.document] is a target for
 // the call [:obj.document:] in the [main] method. Therefore, dart2js
 // would not compile [A.document].
 
 class A {
   get document => 42;
 }
 
 abstract class B {
-  abstract get document;
+  get document;  // Abstract.
 }
 
 class C extends A implements B {
 }
 
 int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 
 void main() {
   var tab = [new Object(), new C()];
   var obj = tab[inscrutable(1)];
   int res = 0;
   if (obj is B) res = obj.document;
   Expect.equals(42, res);
 }