Fix source maps test (failed in checked mode).
Fix source maps test (failed in checked mode).

R=jmesserly@google.com

Review URL: https://codereview.chromium.org//22762003

git-svn-id: cc4dd9cb38464c4587d28ee9e76805c5809f114b@25987 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/source_maps