metatest: Update min version of unittest to support development

lib/metatest.dart

Index: lib/metatest.dart
diff --git a/lib/metatest.dart b/lib/metatest.dart
index c8b5660a9bc750ce331d9452322bb18c27ee3ccf..d037a0e1b9e4d913eebf02657ab257f8ec8be0fe 100644
--- a/lib/metatest.dart
+++ b/lib/metatest.dart
@@ -214,7 +214,6 @@ String _summarizeTests(Map results) {
 
   buffer.writeln();
 
-  var success = false;
   if (results['passed'] == 0 && results['failed'] == 0 &&
       results['errors'] == 0 && results['uncaughtError'] == null) {
     buffer.write('No tests found.');
@@ -222,7 +221,6 @@ String _summarizeTests(Map results) {
   } else if (results['failed'] == 0 && results['errors'] == 0 &&
       results['uncaughtError'] == null) {
     buffer.write('All ${results['passed']} tests passed.');
-    success = true;
   } else {
     if (results['uncaughtError'] != null) {
       buffer.write('Top-level uncaught error: ${results['uncaughtError']}');