Conceptually this is a port of analyzer/nullable_type_inference.dart.
It contains some refactoring and other improvements, as well as tests.
Change-Id: I448440793f11e2e87583d9d47c0e6a1e54ae21f7
Reviewed-on: https://dart-review.googlesource.com/34308
Commit-Queue: Jenny Messerly <jmesserly@google.com>
Reviewed-by: Vijay Menon <vsm@google.com>