Split unittest tests into separate test files.

pkg/unittest/test/unittest_correct_callback_test.dart

+// Copyright (c) 2013, 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.
+
+library unittestTest;
+import 'dart:isolate';
+import 'dart:async';
+import 'package:unittest/unittest.dart';
+
+part 'unittest_test_utils.dart';
+
+var testName = 'correct callback test';
+
+var testFunction = (TestConfiguration testConfig) {
+  test(testName,
+      () =>_defer(expectAsync0((){ ++testConfig.count;})));
+};
+
+var expected = buildStatusString(1, 0, 0, testName, count: 1);