Don't allow a #source directive in a library unit include itself

tests/language/src/SourceSelfNegativeTest.dart

Index: tests/language/src/SourceSelfNegativeTest.dart
diff --git a/tests/language/src/SourceSelfNegativeTest.dart b/tests/language/src/SourceSelfNegativeTest.dart
new file mode 100644
index 0000000000000000000000000000000000000000..1ed1d30dbac7e13c820e720c9ee52f8421fd0eb0
--- /dev/null
+++ b/tests/language/src/SourceSelfNegativeTest.dart
@@ -0,0 +1,12 @@
+// Copyright (c) 2011, 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.
+
+// Dart test program importing the core library explicitly.
+
+#library("SourceSelfNegativeTest");
+#source("SourceSelfNegativeTest.dart");
+
+main() {
+  print('should not be able to #source a file with directives');
+}