stable null safety release, clean up deps (#105)

Make `.packages` files be more clever about defaults. (#80)

* Make `.packages` files be more clever about defaults.

The `PackageConfig` for a `.packages` file now assigns a default language version of 2.7
to all packages, and if the package location ends in `/lib/`, it assumes the package's
root directory is the parent directory of that.

* Fix test.

* Fix another test.

Make version number ready for release.

Merge pull request #49 from dart-lang/sdk_3

bump sdk constraint to 3.0.0