Fix the flutter SDK search box loading on non-root pages with --use-base-href

[jcollins-g]

Fixes #2157.

This combines the attempts in #2118 and #2125 into something that works for the case in Flutter where they've changed the directory level of everything except the index to one down (under flutter/), and rewritten the href on the index page accordingly. I didn't notice this as the serve-flutter-docs grinder doesn't do that hack and I didn't stop to think about the implications of that at the time... Also, serve-flutter-docs doesn't use --use-base-href so a manual test verifying the search box worked with that wouldn't have helped in any event.

I'm open to a better way of doing this in javascript, but I have tried several alternatives (including trying to read the base tag directly) and this seems to be the one that works reliably.

[coveralls]

Coverage remained the same at 93.564% when pulling 8f4765f on flutter-search-box into c535249 on master.