doc(pick-pr): Remove an obsolete comment
Revert "chore: update pubspec.lock"

This reverts commit a157cb2b6af63f3897692aa6a85e6ce0a15de948.

This commit did not pass Travis.
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