Remove @host and other legacy CSS no longer used

Note about Dartium: Dartium by default doesn't support the newer rules; to turn on the support, the "Experimental Web Platform features" flag must be enabled in chrome://flags.
Fix some incorrectly translated @host -> :host rules
Fox a comment
Merge two CSS rules into one (didn't work before?)
Made :host rules in CSS work in both Dartium and Chrome

Dartium and Chrome currently require different syntax for :host. See bug https://github.com/dart-lang/spark/issues/733 for details.
Merge pull request #732 from ussuri/revert-legacy-css-rules-2