Eliminate some of the flakiness in testing:

pkg/unittest/unittest.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A library for writing dart unit tests.
  *
  * To import this library, specify the relative path to
  * pkg/unittest/unittest.dart.
  *
@@ skipping 299 matching lines @@
   int _expectedCalls;
   int _actualCalls = 0;
   int _testNum;
   TestCase _testCase;
   Function _shouldCallBack;
   Function _isDone;
 
   _init(Function callback, Function shouldCallBack, Function isDone,
        [expectedCalls = 0]) {
     ensureInitialized();
-    assert(_currentTest < _tests.length);
+    assert(_currentTest >= 0 &&
+           _currentTest < _tests.length &&
+           _tests[_currentTest] != null);
     _callback = callback;
     _shouldCallBack = shouldCallBack;
     _isDone = isDone;
     _expectedCalls = expectedCalls;
     _testNum = _currentTest;
     _testCase = _tests[_currentTest];
     if (expectedCalls > 0) {
       _testCase.callbackFunctionsOutstanding++;
     }
   }
@@ skipping 536 matching lines @@
 }
 
 /** Enable a test by ID. */
 void enableTest(int testId) => _setTestEnabledState(testId, true);
 
 /** Disable a test by ID. */
 void disableTest(int testId) => _setTestEnabledState(testId, false);
 
 /** Signature for a test function. */
 typedef void TestFunction();