Add element annotations to index.json

[chalin]

Annotations are being collected and emitted as part of the documentation (#1268), but it would be great if annotations -- at least a subset -- were also put in index.json. A very useful annotation would be @Deprecated().

This is needed for AngularDart API docs:

• Deprecated items in angular/api homepage should be crossed out #929.

Related:

• Display annotations on fields in dartdoc Display annotations on fields in dartdoc #1268

cc @kwalrath