Don't Throw Error when setting unittest config twice

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.
 
 /// Support for writing Dart unit tests.
 ///
 /// For information on installing and importing this library, see the
 /// [unittest package on pub.dartlang.org]
 /// (http://pub.dartlang.org/packages/unittest).
 ///
@@ skipping 176 matching lines @@
   }
   return _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');
+      logMessage('Warning: The unittestConfiguration has already been set. New '
+          'unittestConfiguration ignored.');
+    } else {
+      _config = value;
     }
-    _config = value;
   }
 }
 
 /// Can be called by tests to log status. Tests should use this
 /// instead of [print].
 void logMessage(String message) =>
     _config.onLogMessage(currentTestCase, message);
 
 /// Separator used between group names and test names.
 String groupSep = ' ';
@@ skipping 380 matching lines @@
 ///
 /// This allows for multiple invocations of the unittest library in the same
 /// application instance.
 /// This is useful when, for example, creating a test runner application which
 /// needs to create a new pristine test environment on each invocation to run
 /// a given set of test.
 dynamic withTestEnvironment(callback()) {
   return runZoned(callback,
       zoneValues: {_UNITTEST_ENVIRONMENT: new _TestEnvironment()});
 }