add a chrome pub transformer

Not yet ready for review - still a WIP.

This adds a transformer to re-write `.html` files to use the precompiled.js output from dart2js, instead of the non-CSP output.

Some options are

- to only re-write files that live in a directory with a `manifest.json` file
- tell the user that best-practice is to explicitly specify their entry-points in their pubspec.yaml
- look at some way to replace the foo.dart.js output with the CSP output. Not sure if it's better to re-write the html or swap one output with another. I believe the polymer transformer chooses to re-write the html.