Add the ability to run test suites in parallel.

test/runner/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.
 
 @TestOn("vm")
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:test/src/util/io.dart';
@@ skipping 214 matching lines @@
         five
         six
 
         +0 -1: wait
         +1 -1: wait
         +1 -1: Some tests failed.""");
     });
   });
 }
 
-void _expectReport(String tests, String expected, {List<String> args}) {
+void _expectReport(String tests, String expected, {List<String> args,
+    int concurrency}) {
+  if (concurrency == null) concurrency = 1;
+
   var dart = """
 import 'dart:async';
 
 import 'package:test/test.dart';
 
 void main() {
 $tests
 }
 """;
 
   expect(withTempDir((path) {
     new File(p.join(path, "test.dart")).writeAsStringSync(dart);
     if (args == null) args = [];
     args = args.toList()..add("test.dart");
+    args.add("--concurrency=$concurrency");
     var result = runUnittest(args, workingDirectory: path);
 
     // Convert CRs into newlines, remove excess trailing whitespace, and trim
     // off timestamps.
     var actual = result.stdout.trim().split(new RegExp(r"[\r\n]")).map((line) {
       if (line.startsWith("  ") || line.isEmpty) return line.trimRight();
       return line.trim().replaceFirst(new RegExp("^[0-9]{2}:[0-9]{2} "), "");
     }).join("\n");
 
     // Un-indent the expected string.
     var indentation = expected.indexOf(new RegExp("[^ ]"));
     expected = expected.split("\n").map((line) {
       if (line.isEmpty) return line;
       return line.substring(indentation);
     }).join("\n");
 
     expect(actual, equals(expected));
   }), completes);
 }