fix analysis messages

test/codegen/fieldtest.dart

 // Copyright (c) 2015, 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.
 
 library fieldtest;
 
 class A {
   int x = 42;
 }
 
@@ skipping 13 matching lines @@
   return a.x;
 }
 
 baz(A a) => a.x;
 
 int compute() => 123;
 int y = compute() + 444;
 
 String get q => 'life, ' + 'the universe ' + 'and everything';
 int get z => 42;
-int set z(value) {
+void set z(value) {
   y = value;
 }
 
 void main() {
   var a = new A();
   foo(a);
   bar(a);
   print(baz(a));
 }