regen with new apigen

fixes https://github.com/dart-lang/paper-elements/issues/70
oh weird! :-) Makes sense, though.

I do wonder however if we can do something to keep the element vs mixin namespaces separate/more explicit. For example, here we could say:

```yaml
package_mappings:
  elements:
    - core-: core_elements
    - web-animations: core_elements
  mixins:
    - Core: core_elements
```

I don't think we have to change it now, but maybe we should add a comment to explain that this line is for the mixins