Don't try to measure the running time if the test hasn't started yet.

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].
@@ skipping 140 matching lines @@
     } else {
       _result = testResult;
       _config.onTestResultChanged(this);
     }
   }
 
   void _complete(String testResult,
                 [String messageText = '',
                  String stack = '']) {
     if (runningTime == null) {
-      _runningTime = new DateTime.now().difference(startTime);
+      // The startTime can be `null` if an error happened during setup. In this
+      // case we simply report a running time of 0.
+      if (startTime != null) {
+        _runningTime = new DateTime.now().difference(startTime);
+      } else {
+        _runningTime = const Duration(seconds: 0);
+      }
     }
     _setResult(testResult, messageText, stack);
     if (!_doneTeardown) {
       _doneTeardown = true;
       if (tearDown != null) {
         var rtn = tearDown();
         if (rtn is Future) {
           rtn.then((_) {
             _notifyComplete();
           })
@@ skipping 30 matching lines @@
   void error(String messageText, [String stack = '']) {
     _complete(ERROR, messageText, stack);
   }
 
   void _markCallbackComplete() {
     if (--_callbackFunctionsOutstanding == 0 && !isComplete) {
       pass();
     }
   }
 }