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

pkg/unittest/lib/src/test_case.dart

Index: pkg/unittest/lib/src/test_case.dart
diff --git a/pkg/unittest/lib/src/test_case.dart b/pkg/unittest/lib/src/test_case.dart
index f8eb233530ddb98ad409d0c9db82ed12c344c48c..23d04dee256a733c0e60f2f39d06e10a7ccbf8a4 100644
--- a/pkg/unittest/lib/src/test_case.dart
+++ b/pkg/unittest/lib/src/test_case.dart
@@ -158,7 +158,13 @@ class TestCase {
                 [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) {