Update min SDK to 2.4 (dart-lang/yaml#59)

Related to https://github.com/dart-lang/test/issues/1141

pkgs/yaml/.travis.yml

diff --git a/pkgs/yaml/.travis.yml b/pkgs/yaml/.travis.yml
index 78ba590..a25a59e 100644
--- a/pkgs/yaml/.travis.yml
+++ b/pkgs/yaml/.travis.yml
@@ -1,7 +1,7 @@
 language: dart
 
 dart:
-  - 2.3.0
+  - 2.4.0
   - dev
 
 dart_task:

pkgs/yaml/CHANGELOG.md

diff --git a/pkgs/yaml/CHANGELOG.md b/pkgs/yaml/CHANGELOG.md
index bc0cb78..258fb62 100644
--- a/pkgs/yaml/CHANGELOG.md
+++ b/pkgs/yaml/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 2.2.1
+
+* Update min Dart SDK to `2.4.0`.
+
 ## 2.2.0
 
 * POSSIBLY BREAKING CHANGE: Make `YamlMap` preserve parsed key order.

pkgs/yaml/pubspec.yaml

diff --git a/pkgs/yaml/pubspec.yaml b/pkgs/yaml/pubspec.yaml
index d83ad8a..68f6537 100644
--- a/pkgs/yaml/pubspec.yaml
+++ b/pkgs/yaml/pubspec.yaml
@@ -6,7 +6,7 @@
 homepage: https://github.com/dart-lang/yaml
 
 environment:
-  sdk: '>=2.3.0 <3.0.0'
+  sdk: '>=2.4.0 <3.0.0'
 
 dependencies:
   charcode: ^1.1.0