Fix analyzer warnings in service tests

runtime/observatory/tests/service/gc_test.dart

 // Copyright (c) 2014, 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=--compile-all --error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/service_io.dart';
 import 'test_helper.dart';
 
 import 'dart:async';
 
 void script() {
-  List<int> data;
   var grow;
   grow = (int iterations, int size, Duration duration) {
     if (iterations <= 0) {
       return;
     }
-    data = new List<int>(size);
+    new List<int>(size);
     new Timer(duration, () => grow(iterations - 1, size, duration));
   };
   grow(100, 1 << 24, new Duration(seconds: 1));
 }
 
 var tests = [
 
 (Isolate isolate) {
   Completer completer = new Completer();
   // Expect at least this many GC events.
   int gcCountdown = 3;
   isolate.vm.events.stream.listen((ServiceEvent event) {
     if (event.eventType == ServiceEvent.kGC) {
       print('Received GC event');
       if (--gcCountdown == 0) {
         completer.complete();
       }
     }
   });
   return completer.future;
 },
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: script);