[WIP] 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.


TODO:
- [ ] Refactor ShadowDomComponentFactory#call and TranscludingComponentFactory#call.
- [ ] Test ShimmingViewCache in isolation. At the moment it it tested indirectly. 
- [ ] Set up one set of tests for both shims. Similar how we test the two component factories. 
I'll investigate if it can be used.