Add native extensions to the Dart shell, on the linux platform.

tests/standalone/src/TestExtensionTest.dart

+// Copyright (c) 2012, 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.
+
+#library("test_extension_test");
+
+#import('test_extension.dart');
+
+main() {
+  Expect.equals('cat 13', new Cat(13).toString(), 'new Cat(13).toString()');
+
+  Expect.equals(3, Cat.ifNull(null, 3), 'Cat.ifNull(null, 3)');
+  Expect.equals(4, Cat.ifNull(4, null), 'Cat.ifNull(4, null)');
+  Expect.equals(5, Cat.ifNull(5, 9), 'Cat.ifNull(5, 9)');
+  Expect.isNull(Cat.ifNull(null, null), 'Cat.ifNull(null, null)');
+}