Reverting 12485 and 12480 just for the sake of mocking,

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 275 matching lines @@
   if (_soloTest != null) {
     throw new Exception('Only one test can be soloed right now.');
   }
 
   ensureInitialized();
 
   _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;
@@ skipping 43 matching lines @@
     if (_testCase.isComplete) {
       _testCase.error(
           'Callback called after already being marked as done ($_actualCalls).',
           '');
       return false;
     } else {
       return true;
     }
   }
 
-  invoke([arg0, arg1, arg2, arg3, arg4]) {
+  invoke([arg0 = _sentinel, arg1 = _sentinel, arg2 = _sentinel,
+          arg3 = _sentinel, arg4 = _sentinel]) {
     return guardAsync(() {
       ++_actualCalls;
-      if (!_shouldCallBack()) return;
-      if (!?arg0) return _callback();
-      if (!?arg1) return _callback(arg0);
-      if (!?arg2) return _callback(arg0, arg1);
-      if (!?arg3) return _callback(arg0, arg1, arg2);
-      if (!?arg4) return _callback(arg0, arg1, arg2, arg3);
-      _testCase.error(
-          'unittest lib does not support callbacks with more than'
-          ' 4 arguments.',
-          '');
-      },
-      _after, _testNum);
+      if (!_shouldCallBack()) {
+        return;
+      } else if (arg0 == _sentinel) {
+        return _callback();
+      } else if (arg1 == _sentinel) {
+        return _callback(arg0);
+      } else if (arg2 == _sentinel) {
+        return _callback(arg0, arg1);
+      } else if (arg3 == _sentinel) {
+        return _callback(arg0, arg1, arg2);
+      } else if (arg4 == _sentinel) {
+        return _callback(arg0, arg1, arg2, arg3);
+      } else {
+        _testCase.error(
+           'unittest lib does not support callbacks with more than'
+              ' 4 arguments.',
+           '');
+      }
+    },
+    _after, _testNum);
   }
 
   invoke0() {
     return guardAsync(
         () {
           ++_actualCalls;
           if (_shouldCallBack()) {
             return _callback();
           }
         },
@@ skipping 463 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();