Async initialization for DateFormat

pkg/intl/lib/intl_helpers.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.
+
+/**
+ * A library for general helper code associated with the intl library
+ * rather than confined to specific parts of it.
+ */
+
+#library("intl_helpers");
+
+/**
+ * This is used as a marker for a locale data map that hasn't been initialized,
+ * and will throw an exception on any usage.
+ */
+class UninitializedLocaleData {
+  final String message;
+  const UninitializedLocaleData(this.message);
+
+  operator [](String key) {
+    _throwException();
+  }
+  List getKeys() => _throwException();
+  bool containsKey(String key) => _throwException();
+
+  _throwException() {
+    throw new LocaleDataException("Locale data has not been initialized"
+        ", call $message.");
+  }
+}
+
+class LocaleDataException implements Exception {
+  final String message;
+  LocaleDataException(this.message);
+  toString() => "LocaleDataException: $message";
+}
+
+/**
+ *  An abstract superclass for data readers to keep the type system happy.
+ */
+abstract class LocaleDataReader {
+  abstract Future read(String locale);
+}