Update version to 0.19.0. (#17203)

dev/bots/docs.sh

diff --git a/dev/bots/docs.sh b/dev/bots/docs.sh
index 0e278e9..8fda2c6 100755
--- a/dev/bots/docs.sh
+++ b/dev/bots/docs.sh
@@ -22,7 +22,7 @@
 fi
 
 # Install dartdoc.
-bin/cache/dart-sdk/bin/pub global activate dartdoc 0.18.1
+bin/cache/dart-sdk/bin/pub global activate dartdoc 0.19.0
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc.