Small fixes to package:messages

[mosuem]

From the review in dart-lang/dart-pad#2748

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:messages	0.1.1	ready to publish	messages-v0.1.1
package:messages_builder	0.1.1	ready to publish	messages_builder-v0.1.1
package:intl	0.19.0	already published at pub.dev
package:intl_translation	0.19.0-wip	WIP (no publish necessary)
package:intl4x	0.7.0	already published at pub.dev
package:messages_shrinker	0.1.0	already published at pub.dev
package:messages_serializer	0.1.0	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[devoncarew]

🎉