When direction is dynamically changed by setting the `dir` attribute to `rtl`, styles matching `:dir(rtl)` are applied as expected; however, if the `dir` attribute is removed, styles matching `:dir(rtl)` unexpectedly still apply.

See this reproduction: https://lit.dev/playground/#gist=b7c7e390a923d0a54b5c3211bd69a668

Expected: 
1. check the `rtl` checkbox to set document to rtl mode
2. styles matching `:dir(rtl)` apply
3. uncheck the `rtl` checkbox to set document back to default (ltr) direction
4. styles matching `:dir(rtl)` do *not* apply

Actual: 
1. check the `rtl` checkbox to set document to rtl mode
2. styles matching `:dir(rtl)` apply
3. uncheck the `rtl` checkbox to set document back to default (ltr) direction
4. styles matching `:dir(rtl)` *unexpectedly do* apply