Migrate test block 3 to Dart 2.0.

tests/corelib_2/collection_to_string_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.
 
 /**
  * Tests for the toString methods on collections and maps.
  */
 
 library collection_to_string;
 
@@ skipping 367 matching lines @@
   return rand.nextBool();
 }
 
 /** Returns the alphabetized characters in a string. */
 String alphagram(String s) {
   // Calling [toList] to convert unmodifiable list to normal list.
   List<int> chars = s.codeUnits.toList();
   chars.sort((int a, int b) => a - b);
   return new String.fromCharCodes(chars);
 }