pkg/unittest: remove overriding stack variable

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//58903013

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/unittest@30322 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/test_case.dart

@@ -71,7 +71,6 @@ class TestCase {
   bool get isComplete => !enabled || result != null;
 
   Function _errorHandler(String stage) => (e, stack) {
-    var stack;
     if (stack == null && e is Error) {
       stack = e.stackTrace;
     }