Add missing abstract modifier.

tests/language/vm/regress_licm_test.dart

 // Copyright (c) 2017, 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.
 //
 // VMOptions=--enable-inlining-annotations --optimization-counter-threshold=1000 --no-background-compilation
 
 // Regression test for correct LICM and type propagation.
 
 const AlwaysInline = "AlwaysInline";
 const NeverInline = "NeverInline";
 
 class Attribute {
   final id = 123;
 }
 
-class Name {
+abstract class Name {
   Name(this.name);
   final String name;
   get attr;
 
   @AlwaysInline
   int compareTo(other) {
     int nameCompare = name.compareTo(other.name);
     if (nameCompare != 0) return nameCompare;
     if (attr == null) return 0;
     return attr.id - other.attr.id;
@@ skipping 44 matching lines @@
               ];
 
   find(list, new AName());
   find(list, new BName("e"));
   find(list, new BName("b"));
   for (var i = 0; i < 1000; ++i) {
     find(list, new BName("b"));
     find(list, new BName("e"));
   }
 }