* Normalize paths before they're passed to dart2js. It handles "." and
  ".." path components poorly.

* Ensure that paths emitted by list-package-dirs are always absolute and
  normalized for backwards-compatibility with the existing protocol.

* Make Package.listFiles work for an in-memory package.

* Fix an error message expectation in packages_file_test.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1228683003 .