Add language tests for Issue 18628.

tools/testing/dart/test_options.dart

Index: tools/testing/dart/test_options.dart
diff --git a/tools/testing/dart/test_options.dart b/tools/testing/dart/test_options.dart
index 7d5540b16e9f17b2b4ea800ffb839abc6cce77d7..572329ef45780c46069a80dc07be7cc3dc8483fd 100644
--- a/tools/testing/dart/test_options.dart
+++ b/tools/testing/dart/test_options.dart
@@ -810,7 +810,13 @@ Note: currently only implemented for dart2js.''',
    * Print out usage information.
    */
   void _printHelp() {
-    print('usage: dart test.dart [options]\n');
+    print('usage: dart test.dart [options] [selector]');
+    print('');
+    print('The optional selector limits the tests that will be run.');
+    print('For example, the selector "language/issue", or equivalently');
+    print('"language/*issue*", limits to test files matching the regexp');
+    print('".*issue.*\\.dart" in the "tests/language" directory.');
+    print('');
     print('Options:\n');
     for (var option in _options) {
       print('${option.name}: ${option.description}.');