Add support for running browser tests to the test runner.

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.
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:unittest/src/util/exit_codes.dart' as exit_codes;
 import 'package:unittest/unittest.dart';
 
@@ skipping 18 matching lines @@
 
 void main() {
   test("failure", () => throw new TestFailure("oh no"));
 }
 """;
 
 final _usage = """
 Usage: pub run unittest:unittest [files or directories...]
 
 -h, --help          Shows this usage information.
+-p, --platform      The platform(s) on which to run the tests.
+                    [vm (default), chrome]
+
     --[no-]color    Whether to use terminal colors.
                     (auto-detected by default)
 """;
 
 void main() {
   setUp(() {
     _sandbox = Directory.systemTemp.createTempSync('unittest_').path;
   });
 
   tearDown(() {
@@ skipping 177 matching lines @@
       expect(result.stdout, contains("\u001b[31m"));
     });
   });
 }
 
 ProcessResult _runUnittest(List<String> args) =>
     runUnittest(args, workingDirectory: _sandbox);
 
 ProcessResult _runDart(List<String> args) =>
     runDart(args, workingDirectory: _sandbox);