Make mappedBy lazy.

pkg/intl/test/date_time_format_file_test_stub.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 date formatting and parsing using locale data read from the
  * local file system.
  */
 
 library date_time_format_file_test;
@@ skipping 10 matching lines @@
   // Initialize one locale just so we know what the list is.
   test('Run all date formatting tests with locales from files', () {
     initializeDateFormatting("en_US", dataDirectory).then(
         expectAsync1((_) => runEverything(getSubset)));
   });
 }
 
 void runEverything(Function getSubset) {
   // Initialize all locales and wait for them to finish before running tests.
   var futures = DateFormat.allLocalesWithSymbols().mappedBy(
-      (locale) => initializeDateFormatting(locale, dataDirectory));
+      (locale) => initializeDateFormatting(locale, dataDirectory))
+      .toList();
   test('Run all date formatting tests nested test', () {
     Futures.wait(futures).then(
         expectAsync1((results) => runDateTests(getSubset())));
   });
 }