feat(transformers): Expression extractor transformer URI improvements

- Adding support for resolving 'packages/' uris (previously only handled '/packages/') from annotation templateUris.
- Adding support for /packages/ uris to the html_files transformer parameter for referencing package-relative URIs.
- Generating warnings for not-found files rather than build errors.

Closes #879
fix: expose missing NgShadowRootAware
chore(dart2js): Re-enable tests that were disabled because of V8 bug in Chrome 33.

Closes #878