Splitting this from #5930 since it's separate to the fix for handling null properly in Dart-Code.

It appears that evt.test.root_url is undefined but evt.test.url is null, and maybeUriToFilePath handles undefined but not null.

{"seq":2273,"type":"event","body":{"test":{"id":5,"name":"AddStaticTest test_multipleFields","suiteID":0,"groupIDs":[2,3],"metadata":{"skip":true,"skipReason":"does not have \"solo\""},"line":null,"column":null,"url":null},"type":"testStart","time":1055},"event":"dart.testNotification"}

It's not clear to me why we would have nulls here instead of the actual location of the test. Could it be a bug upstream?