Fix caching in web platform

The first commit fix the caching in `fromHtml()`: the `viewFactory`  instances were stored and retrieved under a different key. This made the cache inoperative for cases where shim is needed.

The second commit fix the caching in `fromUrl()`: the `viewFactory` instances were stored under a key that do not depend on the selector name. This would have resulted in getting a wrong shim when:
- two component share a same `templateUrl`,
- a component has multiple different `selector`s .