Reverting 12485 and 12480 just for the sake of mocking,

pkg/unittest/unittest.dart

Index: pkg/unittest/unittest.dart
===================================================================
--- pkg/unittest/unittest.dart	(revision 12486)
+++ pkg/unittest/unittest.dart	(working copy)
@@ -293,6 +293,15 @@
   _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).
@@ -356,21 +365,30 @@
     }
   }
 
-  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() {