Fix static type warnings in test.dart.

tools/testing/dart/test_options.dart

 // Copyright (c) 2011, 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("test_options_parser");
 
 List<String> defaultTestSelectors =
     const ['dartc', 'samples', 'standalone', 'corelib', 'co19', 'language',
            'isolate', 'stub-generator', 'vm', 'client'];
 
@@ skipping 388 matching lines @@
   void _printHelp() {
     print('usage: dart test.dart [options]\n');
     print('Options:\n');
     for (var option in _options) {
       print('${option.name}: ${option.description}.');
       for (var name in option.keys) {
         assert(name.startsWith('-'));
         var buffer = new StringBuffer();;
         buffer.add(name);
         if (option.type == 'bool') {
-          assert(option.values.empty());
+          assert(option.values.isEmpty());
         } else {
           buffer.add(name.startsWith('--') ? '=' : ' ');
           if (option.type == 'int') {
-            assert(option.values.empty());
+            assert(option.values.isEmpty());
             buffer.add('n (default: ${option.defaultValue})');
           } else {
             buffer.add('[');
             bool first = true;
             for (var value in option.values) {
               if (!first) buffer.add(", ");
               if (value == option.defaultValue) buffer.add('*');
               buffer.add(value);
               first = false;
             }
@@ skipping 15 matching lines @@
       if (option.keys.some((key) => key == name)) {
         return option;
       }
     }
     return null;
   }
 
 
   List<_TestOptionSpecification> _options;
 }