Eliminate explicit abstract from class members in tests

tests/language/external_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.
 
 class Foo {
   var x;
   f() {}
 
   external var x01;  /// 01: compile-time error
   external int x02;  /// 02: compile-time error
 
   external f10();   /// 10: runtime error
   external f11() { }  /// 11: compile-time error
   external f12() => 1;  /// 12: compile-time error
   external static f13();  /// 13: runtime error
   static external f14();  /// 14: compile-time error
-  external abstract f15();  /// 15: compile-time error
   int external f16();  /// 16: compile-time error
 
   external Foo.n20();  /// 20: runtime error
   external Foo.n21() : x(1);  /// 21: compile-time error
   external Foo.n22() { x = 1; }  /// 22: compile-time error
   external factory Foo.n23() => new Foo();  /// 23: compile-time error
 }
 
 external int t06(int i) { }  /// 30: compile-time error
 external int t07(int i) => i + 1;  /// 31: compile-time error
@@ skipping 20 matching lines @@
 
   // Try calling an unpatched external constructor.
   try {                                               /// 20: continued
     var foo = new Foo.n09();                          /// 20: continued
   } on String catch (exc) {                           /// 20: continued
     if (exc == "External implementation missing.") {  /// 20: continued
       throw exc;                                      /// 20: continued
     }                                                 /// 20: continued
   }                                                   /// 20: continued
 }