Remove throw from unittest.

pkg/unittest/lib/unittest.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * A library for writing dart unit tests.
  *
  * To import this library, install the
  * [unittest package](http://pub.dartlang.org/packages/unittest) via the pub
  * package manager. See the [Getting Started](http://pub.dartlang.org/doc)
@@ skipping 158 matching lines @@
 
 /** [Configuration] used by the unittest library. */
 Configuration get unittestConfiguration => _config;
 
 /**
  * Sets the [Configuration] used by the unittest library.
  *
  * 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);
 
 /**
  * Description text of the current test group. If multiple groups are nested,
  * this will contain all of their text concatenated.
  */
 String _currentGroup = '';
 
@@ skipping 652 matching lines @@
 }
 
 /** Enable a test by ID. */
 void enableTest(int testId) => _setTestEnabledState(testId, true);
 
 /** Disable a test by ID. */
 void disableTest(int testId) => _setTestEnabledState(testId, false);
 
 /** Signature for a test function. */
 typedef dynamic TestFunction();