Add firefox support.

test/runner/runner_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")
 
 import 'dart:io';
 import 'dart:math' as math;
 
 import 'package:path/path.dart' as p;
@@ skipping 29 matching lines @@
 
 final _usage = """
 Usage: pub run test:test [files or directories...]
 
 -h, --help                     Shows this usage information.
 -n, --name                     A substring of the name of the test to run.
                                Regular expression syntax is supported.
 
 -N, --plain-name               A plain-text substring of the name of the test to run.
 -p, --platform                 The platform(s) on which to run the tests.
-                               [vm (default), chrome]
+                               [vm (default), chrome, firefox]
 
 -j, --concurrency=<threads>    The number of concurrent test suites run.
                                (defaults to $_defaultConcurrency)
 
     --pub-serve=<port>         The port of a pub serve instance serving "test/".
     --[no-]color               Whether to use terminal colors.
                                (auto-detected by default)
 """;
 
 void main() {
@@ skipping 317 matching lines @@
       });
     });
   });
 }
 
 ProcessResult _runUnittest(List<String> args) =>
     runUnittest(args, workingDirectory: _sandbox);
 
 ProcessResult _runDart(List<String> args) =>
     runDart(args, workingDirectory: _sandbox);