Re-enable throwing if we try to set the unit test configuration twice.

pkg/unittest/lib/unittest.dart

Index: pkg/unittest/lib/unittest.dart
diff --git a/pkg/unittest/lib/unittest.dart b/pkg/unittest/lib/unittest.dart
index efd2e83bea79e15ac99bb8178d6624e20b4249c8..329e20dc68b61be922a967f566b95ca5635b83e1 100644
--- a/pkg/unittest/lib/unittest.dart
+++ b/pkg/unittest/lib/unittest.dart
@@ -176,7 +176,12 @@ Configuration get unittestConfiguration => _config;
  * Throws a [StateError] if there is an existing, incompatible value.
  */
 void set unittestConfiguration(Configuration value) {
-  _config = value;
+  if(!identical(_config, value)) {
+    if(_config != null) {
+      throw new StateError('unittestConfiguration has already been set');
+    }
+    _config = value;
+  }
 }
 
 void logMessage(String message) => _config.logMessage(message);