Split unittest tests into separate test files.

pkg/unittest/test/unittest_testcases_immutable_test.dart

Index: pkg/unittest/test/unittest_testcases_immutable_test.dart
===================================================================
--- pkg/unittest/test/unittest_testcases_immutable_test.dart	(revision 0)
+++ pkg/unittest/test/unittest_testcases_immutable_test.dart	(revision 0)
@@ -0,0 +1,21 @@
+// 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 = 'testcases immutable';
+
+var testFunction = (_) {
+  test(testName, () {
+    expect(() => testCases.clear(), throwsUnsupportedError);
+    expect(() => testCases.removeLast(), throwsUnsupportedError);
+  });
+};
+
+var expected = buildStatusString(1, 0, 0, testName);