Mark pub missing_file_test flaky.

BUG=
R=pquitslund@google.com

Review URL: https://codereview.chromium.org//22889013

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@26371 260f80e4-7a28-3924-810f-c04153c831b5

pub.status

@@ -9,3 +9,5 @@
 # Pub only runs on the standalone VM, not the browser.
 [ $runtime == drt || $runtime == dartium || $runtime == opera ]
 *: Skip
+
+test/serve/missing_file_test: Pass, Fail # 12570