Expose HTTP handler for `webdev serve` as library

When writing an HTTP server in Dart and using `webdev` for building assets, it would be nice to have an HTTP handler that:
A) serves what `webdev serve` serves when running in development mode, and,
B) serves what is built by `webdev build` when running in production mode.

This probably means depending on `webdev` as a `dependency` (not `dev_dependency`), and using it as a library if running in development mode.

Use case: https://github.com/dart-lang/appengine/pull/92#issuecomment-470146418

Start by reviewing the webdev serve and webdev build entry points and how their development and production asset-serving paths are organized. Define the library-facing HTTP handler and dependency arrangement so both modes expose the requested behavior, then add or update tests covering development and production serving.

Written by the indexing model from the issue text.