Make field final in const class.

tests/language/switch_this_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.
 
 // Test comparison of this and a constant.
 
 class Foo {
   static final C1 = const Foo('C1');
   static final C2 = const Foo('C2');
   static final C3 = const Foo('C3');
 
-  var name;
+  final name;
   const Foo(this.name);
 
   foo() {
     switch (this) {
       case C1: return C1;
       case C2: return C2;
       default: return null;
     }
   }
 }
 
 main() {
   Expect.equals(Foo.C1, Foo.C1.foo());
   Expect.equals(Foo.C2, Foo.C2.foo());
   Expect.equals(null, Foo.C3.foo());
 }