Update version and add changelog for null safety (#33)

diff --git a/.travis.yml b/.travis.yml
index 6bd2b01..b634f67 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -28,7 +28,7 @@
 # Only building master means that we don't run two builds for each pull request.
-  only: [master, null_safety]
+  only: [master]
diff --git a/CHANGELOG.md b/CHANGELOG.md
index d3b3410..37e25fd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,14 @@
+## 2.2.0-nullsafety
+Pre-release for the null safety migration of this package.
+Note that 2.2.0 may not be the final stable null safety release version, we
+reserve the right to release it as a 3.0.0 breaking change.
+This release will be pinned to only allow pre-release sdk versions starting from
+2.10.0-2.0.dev, which is the first version where this package will appear in the
+null safety allow list.
 ## 2.1.1
  * Fixed a DDC compilation regression for consumers using the Dart 1.x SDK that was introduced in `2.1.0`.
diff --git a/pubspec.yaml b/pubspec.yaml
index ff0eae6..5897810 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: convert
-version: 2.1.2-dev
+version: 2.2.0-nullsafety
 description: Utilities for converting between data representations.
 homepage: https://github.com/dart-lang/convert