The video.* tests have been failing recently:

2018/08/17 07:45:01 Started test video.PlayVP8
2018/08/17 07:45:01 [07:45:00.679] Restarting ui job
2018/08/17 07:45:02 [07:45:01.833] Waiting for org.chromium.SessionManager D-Bus service
2018/08/17 07:45:02 [07:45:01.846] Asking session_manager to enable Chrome testing
2018/08/17 07:45:02 [07:45:01.848] Waiting for Chrome to write its debugging port to /home/chronos/DevToolsActivePort
2018/08/17 07:45:03 [07:45:02.695] Removing cryptohome for testuser@gmail.com
2018/08/17 07:45:03 [07:45:02.759] Finding OOBE DevTools target
2018/08/17 07:45:03 [07:45:02.776] Connecting to Chrome at ws://127.0.0.1:55552/devtools/page/CC8CE7C3AC279D123919F71268B862E2
2018/08/17 07:45:03 [07:45:02.918] Waiting for OOBE
2018/08/17 07:45:05 [07:45:04.180] Logging in as user "testuser@gmail.com"
2018/08/17 07:45:05 [07:45:04.216] Waiting for cryptohome /home/user/1642ddab45c0d09938a0b983bd6bf1b407205b56
2018/08/17 07:45:06 [07:45:05.120] Waiting for OOBE to be dismissed
2018/08/17 07:45:07 [07:45:06.593] Creating new page with URL http://127.0.0.1:39120/video.html
2018/08/17 07:45:07 [07:45:06.620] Error at state.go:134: Test data "/video.html" wasn't declared in definition passed to testing.AddTest
2018/08/17 07:45:07 [07:45:06.620] Stack trace:
chromiumos/tast/testing.(*State).DataPath(0xc420202000, 0xc4202b4c44, 0xb, 0xc420053c18, 0x0)
	/build/tricky/tmp/portage/chromeos-base/tast-local-tests-cros-0.0.1-r138/work/tast-local-tests-cros-0.0.1/tast-base/src/chromiumos/tast/testing/state.go:134 +0x1e1
chromiumos/tast/local/bundles/cros/video/play.(*dataDir).Open(0xc420202000, 0xc4202b4c44, 0xb, 0xc420053c00, 0x7f260af6111f, 0xc4200d8800, 0xc42007f800)
	/build/tricky/tmp/portage/chromeos-base/tast-local-tests-cros-0.0.1-r138/work/tast-local-tests-cros-0.0.1/src/chromiumos/tast/local/bundles/cros/video/play/play.go:24 +0x41
net/http.serveFile(0x7f260b58ef60, 0xc42028a000, 0xc4203de000, 0x7f260b588200, 0xc420202000, 0xc4202b4c44, 0xb, 0xc420486e01)
	/usr/lib/go/x86_64-cros-linux-gnu/src/net/http/fs.go:551 +0xb7
net/http.(*fileHandler).ServeHTTP(0xc4202a2100, 0x7f260b58ef60, 0xc42028a000, 0xc4203de000)
	/usr/lib/go/x86_64-cros-linux-gnu/src/net/http/fs.go:715 +0xa3
net/http.serverHandler.ServeHTTP(0xc420322340, 0x7f260b58ef60, 0xc42028a000, 0xc4203de000)
	/usr/lib/go/x86_64-cros-linux-gnu/src/net/http/server.go:2694 +0xbe
net/http.(*conn).serve(0xc42001d9a0, 0x7f260b58f320, 0xc4202ce3c0)
	/usr/lib/go/x86_64-cros-linux-gnu/src/net/http/server.go:1830 +0x653
created by net/http.(*Server).Serve
	/usr/lib/go/x86_64-cros-linux-gnu/src/net/http/server.go:2795 +0x27d

I think that the problem is that the http.FileSystem interface's Open function can receive paths with a leading slash, which test.DataPath doesn't expect. I suspect that updating our Open implementation to strip the slash will fix this.