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

tests/language/src/SourceSelfNegativeTest.dart

+// 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.

[jbrosenberg, 2011/10/11 21:32:32]

This test was suggested by Steve Messick

[jbrosenberg, 2011/10/11 22:05:13]

Sorry, it was Dan Rubel who suggested this.

+
+#library("SourceSelfNegativeTest");
+#source("SourceSelfNegativeTest.dart");
+
+main() {
+  print('should not be able to #source a file with directives');
+}