Add an option to run skipped tests.

test/utils.dart

Index: test/utils.dart
diff --git a/test/utils.dart b/test/utils.dart
index 03d457d8ef73b1d69f29b616bdf6fdad34c796ba..5bbba17b936c919b0f4919c2af68c00e532c4c60 100644
--- a/test/utils.dart
+++ b/test/utils.dart
@@ -305,9 +305,9 @@ List<GroupEntry> declare(void body()) {
 }
 
 /// Runs [body] with a declarer and returns an engine that runs those tests.
-Engine declareEngine(void body()) {
+Engine declareEngine(void body(), {bool runSkipped: false}) {
   var declarer = new Declarer()..declare(body);
   return new Engine.withSuites([
     new RunnerSuite(const PluginEnvironment(), declarer.build())
-  ]);
+  ], runSkipped: runSkipped);
 }