Add language tests for Issue 18628.

tools/test.dart

Index: tools/test.dart
diff --git a/tools/test.dart b/tools/test.dart
index 2dcad8168a214808929a141e23e69b030c177ca2..044341449282d713b956bfc96cb29b674f099971 100755
--- a/tools/test.dart
+++ b/tools/test.dart
@@ -18,8 +18,12 @@
  * and passses along all command line arguments to this script.)
  *
  * The command line args of this script are documented in
- * "tools/testing/test_options.dart".
+ * "tools/testing/dart/test_options.dart"; they are printed
+ * when this script is run with "--help".
  *
+ * The default test directory layout is documented in
+ * "tools/testing/dart/test_suite.dart", above
+ * "factory StandardTestSuite.forDirectory".
  */
 
 library test;