chore(travis): skip dart version 1.7.0-dev.1.0 / dartbug 20896
chore(travis): remove dart version blacklist for 1.6.0-dev.8.0

Closes #1350
chore(travis): remove blacklist for g3v1x with sdk 1.5.8
feat(components): implement css encapsulation for transcluding components

* Extract ComponentCssLoader so it can be shared by ShadowDomComponentFactory and TranscludingComponentFactory
* Provide a built-in implementation of WebPlatformShim using css_shim
* Implement a CSS transformer similar to the one provided by Platform.JS
* Rename EventHandler into ShadowBoundary