Add custom initialization of DateTime symbols/patterns

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172142246
diff --git a/test/date_time_format_custom_test.dart b/test/date_time_format_custom_test.dart
new file mode 100644
index 0000000..cdcd638
--- /dev/null
+++ b/test/date_time_format_custom_test.dart
@@ -0,0 +1,29 @@
+// Copyright (c) 2017, 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 date formatting and parsing using custom locale data, which we get
+/// from the local copy.
+
+import 'dart:async';
+import 'date_time_format_test_stub.dart';
+import 'package:intl/date_symbol_data_local.dart' as localSymbols;
+import 'package:intl/date_time_patterns.dart' as localPatterns;
+import 'package:intl/date_symbol_data_custom.dart';
+
+main() {
+  var symbols = localSymbols.dateTimeSymbolMap();
+  var patterns = localPatterns.dateTimePatternMap();
+  List<String> locales = symbols.keys.toList().take(10).toList();
+  // Force inclusion of locales that are hard-coded in tests.
+  var requiredLocales = ["en_US", "de", "fr", "ja", "el", "de_AT"];
+  locales.addAll(requiredLocales);
+  for (var locale in locales) {
+    print("initializing $locale");
+    initializeDateFormattingCustom(
+        locale: locale, symbols: symbols[locale], patterns: patterns[locale]);
+  }
+  runWith(() => locales, null, nullInitialization);
+}
+
+Future nullInitialization(String a, String b) => new Future.value(null);