Support for package: uri scheme for components
There's an interesting long term question about whether package: makes sense as a URI scheme. But assuming it does, it seems sensible to support it for components too.

Some discussion in this issue: https://github.com/dart-lang/web-ui/issues/115

Review URL: https://chromiumcodereview.appspot.com//12463024
Add ChangeRecord and observeChanges API.
Observable collections track INSERT/REMOVE/INDEX operations separately.

Review URL: https://chromiumcodereview.appspot.com//12316058
merged