update pubspec/changelog for 1.2.4

pkgs/source_span/CHANGELOG.md

diff --git a/pkgs/source_span/CHANGELOG.md b/pkgs/source_span/CHANGELOG.md
index d1bcda5..6da8487 100644
--- a/pkgs/source_span/CHANGELOG.md
+++ b/pkgs/source_span/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 1.2.4
+
+* Fix a new strong mode error.
+
 # 1.2.3
 
 * Fix a bug where a point span at the end of a file without a trailing newline

pkgs/source_span/pubspec.yaml

diff --git a/pkgs/source_span/pubspec.yaml b/pkgs/source_span/pubspec.yaml
index 3ef97ed..9e41fdd 100644
--- a/pkgs/source_span/pubspec.yaml
+++ b/pkgs/source_span/pubspec.yaml
@@ -1,5 +1,5 @@
 name: source_span
-version: 1.2.3
+version: 1.2.4
 author: Dart Team <misc@dartlang.org>
 description: A library for identifying source spans and locations.
 homepage: https://github.com/dart-lang/source_span