Add a flag to control the use of color in the test runner.

test/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 14 matching lines @@
 final _failure = """
 import 'dart:async';
 
 import 'package:unittest/unittest.dart';
 
 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.
+    --[no-]color    Whether to use terminal colors.
+                    (auto-detected by default)
+""";
+
 void main() {
   setUp(() {
     _sandbox = Directory.systemTemp.createTempSync('unittest_').path;
   });
 
   tearDown(() {
     new Directory(_sandbox).deleteSync(recursive: true);
   });
 
   test("prints help information", () {
     var result = _runUnittest(["--help"]);
     expect(result.stdout, equals("""
 Runs tests in this package.
 
-Usage: pub run unittest:unittest [files or directories...]
-
--h, --help    Shows this usage information.
-"""));
+$_usage"""));
     expect(result.exitCode, equals(exit_codes.success));
   });
 
   group("fails gracefully if", () {
     test("an invalid option is passed", () {
       var result = _runUnittest(["--asdf"]);
       expect(result.stderr, equals("""
 Could not find an option named "asdf".
 
-Usage: pub run unittest:unittest [files or directories...]
-
--h, --help    Shows this usage information.
-"""));
+$_usage"""));
       expect(result.exitCode, equals(exit_codes.usage));
     });
 
     test("a non-existent file is passed", () {
       var result = _runUnittest(["file"]);
       expect(result.stderr, equals('Failed to load "file": Does not exist.\n'));
       expect(result.exitCode, equals(exit_codes.data));
     });
 
     test("the default directory doesn't exist", () {
@@ skipping 132 matching lines @@
 
     test("directly", () {
       new File(p.join(_sandbox, "test.dart")).writeAsStringSync(_failure);
       var result = _runDart([
         "--package-root=${p.join(packageDir, 'packages')}",
         "test.dart"
       ]);
       expect(result.stdout, contains("Some tests failed."));
     });
   });
+
+  group("flags", () {
+    test("with the --color flag, uses colors", () {
+      new File(p.join(_sandbox, "test.dart")).writeAsStringSync(_failure);
+      var result = _runUnittest(["--color", "test.dart"]);
+      // This is the color code for red.
+      expect(result.stdout, contains("\u001b[31m"));
+    });
+  });
 }
 
 ProcessResult _runUnittest(List<String> args) =>
     runUnittest(args, workingDirectory: _sandbox);
 
 ProcessResult _runDart(List<String> args) =>
     runDart(args, workingDirectory: _sandbox);