Add support for CSS encapsulation to transcluding components

WIP [DO NOT MERGE]

1. Extract ComponentCssLoader so it can be shared by ShadowDomComponentFactory and TranscludingComponentFactory.
2. Implement a simple CSS transformer similar to the one provided by Platform.JS. This is here for completeness. A more full-featured compiler will be submitted as a separate PR.
3. Rename EventHandler into ShadowBoundary.
4. Rename WebPlatform into WebPlatformShim and provide two implementations: one based on platform.js and the other one built into Angular.