Prepare yaml_edit version 2.0.0 (dart-lang/yaml_edit#126)

pkgs/yaml_edit/CHANGELOG.md

diff --git a/pkgs/yaml_edit/CHANGELOG.md b/pkgs/yaml_edit/CHANGELOG.md
index 90746ef..2040c50 100644
--- a/pkgs/yaml_edit/CHANGELOG.md
+++ b/pkgs/yaml_edit/CHANGELOG.md
@@ -1,3 +1,9 @@
+## v2.0.0
+- Migrated to null-safety.
+- API will no-longer return `null` in-place of a `YamlNode`, instead a
+  `YamlNode` with `YamlNode.value == null` should be used. These are easily
+  created with `wrapAsYamlNode(null)`.
+
 ## v1.0.3
 
 - Fixed bug in adding an empty map as a map value.

pkgs/yaml_edit/pubspec.yaml

diff --git a/pkgs/yaml_edit/pubspec.yaml b/pkgs/yaml_edit/pubspec.yaml
index 91ef188..8a98560 100644
--- a/pkgs/yaml_edit/pubspec.yaml
+++ b/pkgs/yaml_edit/pubspec.yaml
@@ -1,5 +1,5 @@
 name: yaml_edit
-version: 1.0.3
+version: 2.0.0
 description: A library for YAML manipulation with comment and whitespace preservation.
 homepage: https://github.com/google/dart-neats/tree/master/yaml_edit
 repository: https://github.com/google/dart-neats.git
@@ -14,4 +14,4 @@
   pedantic: ^1.9.0
   test: ^1.14.4
 environment:
-  sdk: ">=2.12.0-0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"