Mark `when` strong-mode clean.

.analysis_options

diff --git a/.analysis_options b/.analysis_options
new file mode 100644
index 0000000..a10d4c5
--- /dev/null
+++ b/.analysis_options
@@ -0,0 +1,2 @@
+analyzer:
+  strong-mode: true

.gitignore

diff --git a/.gitignore b/.gitignore
index ee1239e..8277286 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
 # Don’t commit the following directories created by pub.
 build/
 packages
+.packages
 .buildlog
 
 # Or settings