Fix memory leak in unittest - the testCases variable doesn't release the

pkg/unittest/lib/src/test_case.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.
 
 part of unittest;
 
 /// Represents the state for an individual unit test.
 ///
 /// Create by calling [test] or [solo_test].
 class TestCase {
   /// Identifier for this test.
   final int id;
 
   /// A description of what the test is specifying.
   final String description;
 
   /// The setup function to call before the test, if any.
-  final Function _setUp;
+  Function _setUp;
 
   /// The teardown function to call after the test, if any.
-  final Function _tearDown;
+  Function _tearDown;
 
   /// The body of the test case.
-  final TestFunction _testFunction;
+  TestFunction _testFunction;
 
   /// Remaining number of callbacks functions that must reach a 'done' state
   /// to wait for before the test completes.
   int _callbackFunctionsOutstanding = 0;
 
   String _message = '';
   /// Error or failure message.
   String get message => _message;
 
   String _result;
@@ skipping 78 matching lines @@
         // Outstanding callbacks exist; we need to return a Future.
         _testComplete = new Completer();
         return _testComplete.future.whenComplete(() {
           if (_tearDown != null) {
             return _tearDown();
           }
         }).catchError(_errorHandler('Teardown'));
       } else if (_tearDown != null) {
         return _tearDown();
       }
-    }).catchError(_errorHandler('Teardown'));
+    }).catchError(_errorHandler('Teardown')).whenComplete(() {
+      _setUp = null;
+      _tearDown = null;
+      _testFunction = null;
+    });
   }
 
   // Set the results, notify the config, and return true if this
   // is the first time the result is being set.
   void _setResult(String testResult, String messageText, StackTrace stack) {
     _message = messageText;
     _stackTrace = getTrace(stack, formatStacks, filterStacks);
     if (_stackTrace == null) _stackTrace = stack;
     if (result == null) {
       _result = testResult;
@@ skipping 44 matching lines @@
   }
 
   void _markCallbackComplete() {
     if (--_callbackFunctionsOutstanding == 0 && !isComplete) {
       _pass();
     }
   }
 
   String toString() => _result != null ? "$description: $result" : description;
 }