Make test.dart exit with an error message when a selector is repeated.

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");
 
 #import("dart:io");
 
 List<String> defaultTestSelectors =
     const ['dartc', 'samples', 'standalone', 'corelib', 'co19', 'language',
@@ skipping 355 matching lines @@
         var slashLocation = pattern.indexOf('/');
         if (slashLocation != -1) {
           suite = pattern.substring(0, slashLocation);
           pattern = pattern.substring(slashLocation + 1);
           pattern = pattern.replaceAll('*', '.*');
           pattern = pattern.replaceAll('/', '.*');
         } else {
           pattern = ".*";
         }
         if (selectorMap.containsKey(suite)) {
-          print("Warning: selector '$suite/$pattern' overrides " +
-                "previous selector for suite '$suite'");
+          print("Error: '$suite/$pattern'.  Only one test selection" +
+                " pattern is allowed to start with '$suite/'");
+          exit(1);
         }
         selectorMap[suite] = new RegExp(pattern);
       }
       configuration['selectors'] = selectorMap;
     }
 
     // Expand the architectures.
     var archs = configuration['arch'];
     if (archs.contains(',')) {
       var result = new List<Map>();
@@ skipping 105 matching lines @@
         return option;
       }
     }
     print('Unknown test option $name');
     exit(1);
   }
 
 
   List<_TestOptionSpecification> _options;
 }