Remove last references to frog from SDK.

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 280 matching lines @@
 
   _soloTest = new TestCase(_tests.length + 1, _fullSpec(spec), body, 0);
   _tests.add(_soloTest);
 }
 
 /** Sentinel value for [_SpreadArgsHelper]. */
 class _Sentinel {
   const _Sentinel();
 }
 
-// TODO(sigmund): make a singleton const field when frog supports passing those
-// as default values to named arguments.
-const _sentinel = const _Sentinel();
-
 /** Simulates spread arguments using named arguments. */
 // TODO(sigmund): remove this class and simply use a closure with named
 // arguments (if still applicable).
 class _SpreadArgsHelper {
   Function _callback;
   int _expectedCalls;
   int _actualCalls = 0;
   int _testNum;
   TestCase _testCase;
   Function _shouldCallBack;
   Function _isDone;
+  static const _sentinel = const _Sentinel();
 
   _init(Function callback, Function shouldCallBack, Function isDone,
        [expectedCalls = 0]) {
     ensureInitialized();
     assert(_currentTest >= 0 &&
            _currentTest < _tests.length &&
            _tests[_currentTest] != null);
     _callback = callback;
     _shouldCallBack = shouldCallBack;
     _isDone = isDone;
@@ skipping 543 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();