Fix issue 24043: do not trust the pattern. Enable a 'hidden' test.

tests/language/string_interpolation_and_buffer_test.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.
 
 // Test to ensure that StringBuffer and string interpolation behaves
 // the same and fail fast.
 
+import "package:expect/expect.dart";
+
 class ToStringWrapper {
   final value;
 
   ToStringWrapper(this.value);
 
   toString() => value;
 }
 
 wrap(value) => new ToStringWrapper(value);
 
@@ skipping 71 matching lines @@
   Expect.equals('Error', buffer([1]));
   Expect.equals('Error', buffer(new Object()));
 
   Expect.equals('Error', initBuffer(null));
   Expect.equals('Success', initBuffer(""));
   Expect.equals('Success', initBuffer("string"));
   Expect.equals('Error', initBuffer([]));
   Expect.equals('Error', initBuffer([1]));
   Expect.equals('Error', initBuffer(new Object()));
 }