Stop making classes implicitly abstract

tests/language/get_set_syntax_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.
 
 var get;
 var get a;            /// 00: compile-time error
 var get b, c;         /// 01: compile-time error
 
 var set;
 var set d;            /// 02: compile-time error
@@ skipping 27 matching lines @@
   abstract List<int> get a2;
   abstract List<int> get a3 => null;  /// 08: compile-time error
   List<int> get b, c; /// 13: compile-time error
 
   List<int> set;
   List<int> set d;    /// 14: compile-time error
   List<int> set e, f; /// 15: compile-time error
 }
 
 main() {
-  new C1();           /// 16: runtime error
+  new C1();
 }