Remove throw from unittest.

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