Add browser tags to all of our tests.

test/runner/browser/firefox_test.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.
 
 @TestOn("vm")
+@Tags(const ["firefox"])
 
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:test/src/runner/browser/firefox.dart';
 
 import '../../io.dart';
 import '../../utils.dart';
 import 'code_server.dart';
 
@@ skipping 59 matching lines @@
 void main() {
   test("failure", () => throw new TestFailure("oh no"));
 }
 """).create();
 
     var test = runTest(["-p", "firefox", "test.dart"]);
     test.stdout.expect(consumeThrough(contains("-1: Some tests failed.")));
     test.shouldExit(1);
   });
 }