Change Duration to take proper named optional parameters.

pkg/unittest/test_case.dart

Index: pkg/unittest/test_case.dart
diff --git a/pkg/unittest/test_case.dart b/pkg/unittest/test_case.dart
index 35e9ff89c23aa86b3472bd7c2cd0aa39ddce2f5c..b0f1d18e12fc78e0cce396606cb8dcd14fe9a93b 100644
--- a/pkg/unittest/test_case.dart
+++ b/pkg/unittest/test_case.dart
@@ -86,7 +86,7 @@ class TestCase {
       // TODO(gram): currently the duration measurement code is blocked
       // by issue 4437. When that is fixed replace the line below with:
       //    runningTime = new Date.now().difference(startTime);
-      runningTime = new Duration(milliseconds:0);
+      runningTime = new Duration(milliseconds: 0);

[bakster, 2012/09/13 09:00:01]

How about introducing a static or a named constructor:
   Duration.empty
instead of using:
   new Duration() and 
   new new Duration(milliseconds: 0)

[kasperl, 2012/09/13 09:04:47]

That's fine but that extra constructor will not make it impossible to use new
Duration() or new Duration(milliseconds: 0), right? Unless we start adding some
kind of weird constraint that you have to pass at least one named argument
somehow.

[Lasse Reichstein Nielsen, 2012/09/13 09:30:46]

Not prevent it, but perhaps it will preempt it by being the better alternative.
E.g.:
 Duration.zero() : this(milliseconds: 0); 
This is more readable, so people will hopefully prefer to use it.

     }
     if (!_doneTeardown) {
       if (_tearDown != null) {