Document date format initialization better and make it possible to use en_US dates without it

pkg/intl/test/date_time_format_uninitialized_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 date formatting and parsing using locale data read from the
+ * local file system.
+ */
+library date_time_format_file_test;
+
+import '../lib/intl.dart';
+import '../lib/date_symbol_data_file.dart';
+import 'date_time_format_test_core.dart';
+import '../../../pkg/unittest/lib/unittest.dart';
+
+main() {
+  runDateTests(['en_US']);
+}