Add support for running browser tests to the test runner.

test/runner/browser/compact_reporter_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/unittest.dart';
+
+import '../../io.dart';
+
+String _sandbox;
+
+void main() {
+  setUp(() {
+    _sandbox = Directory.systemTemp.createTempSync('unittest_').path;
+  });
+
+  tearDown(() {
+    new Directory(_sandbox).deleteSync(recursive: true);
+  });
+
+  test("prints the platform name when running on multiple platforms", () {
+    new File(p.join(_sandbox, "test.dart")).writeAsStringSync("""
+import 'dart:async';
+
+import 'package:path/path.dart' as p;
+import 'package:unittest/unittest.dart';
+
+void main() {
+  test("success", () {});
+}
+""");
+
+    var result = _runUnittest(["-p", "chrome", "-p", "vm", "test.dart"]);
+    expect(result.stdout, contains("[VM]"));
+    expect(result.stdout, contains("[Chrome]"));
+    expect(result.exitCode, equals(0));
+  });
+}
+
+ProcessResult _runUnittest(List<String> args) =>
+    runUnittest(args, workingDirectory: _sandbox);