Add a --reporter flag and expose the expanded reporter.

test/runner/browser/expanded_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.
 
 @TestOn("vm")
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:test/src/util/io.dart';
@@ skipping 17 matching lines @@
 import 'dart:async';
 
 import 'package:path/path.dart' as p;
 import 'package:test/test.dart';
 
 void main() {
   test("success", () {});
 }
 """);
 
-    var result = _runTest(
-        ["-p", "content-shell", "-p", "vm", "-j", "1", "test.dart"]);
+    var result = _runTest([
+      "-r",
+      "expanded",
+      "-p",
+      "content-shell",
+      "-p",
+      "vm",
+      "-j",
+      "1",
+      "test.dart"
+    ]);
     expect(result.stdout, contains("[VM]"));
     expect(result.stdout, contains("[Dartium Content Shell]"));
     expect(result.exitCode, equals(0));
   });
 }
 
 ProcessResult _runTest(List<String> args) =>
     runTest(args, workingDirectory: _sandbox);