Don't Throw Error when setting unittest config twice

pkg/unittest/lib/unittest.dart

Index: pkg/unittest/lib/unittest.dart
===================================================================
--- pkg/unittest/lib/unittest.dart	(revision 42037)
+++ pkg/unittest/lib/unittest.dart	(working copy)
@@ -194,9 +194,11 @@
 void set unittestConfiguration(Configuration value) {
   if (!identical(_config, value)) {
     if (_config != null) {
-      throw new StateError('unittestConfiguration has already been set');
+      logMessage('Warning: The unittestConfiguration has already been set. New '
+          'unittestConfiguration ignored.');
+    } else {
+      _config = value;
     }
-    _config = value;
   }
 }