Implement removeDirectoryTree on POSIX

[brianquinlan]

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:appengine	0.13.11	ready to publish	appengine-v0.13.11
package:gcloud	0.8.19	already published at pub.dev
package:googleapis_firestore_v1	0.1.0-wip	WIP (no publish necessary)
package:native_synchronization	0.3.1	already published at pub.dev

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[devoncarew]

cc @lrhn - it would be nice to be able to refer to character literals ( == 46 could change to something like == c'.').