Adding header rule to FB hosting for API docs.

[RedBrogdon]

Description

This is a modification of the Firebase Hosting config used for https://api.flutter.dev. It adds a CORS Access-Control-Allow-Origin header to the snippets directory naming https://dartpad.dev as an allowed origin. This will allow DartPad to load samples directly from the API Doc server into the browser using client-side Dart.

FWIW, this is the first change to the config since @sethladd set it up three years ago.

See also: https://firebase.google.com/docs/hosting/full-config

Related Issues

None that I know of.

Tests

I have not added any tests, since this is a configuration change for hosting rather than an actual code change.

Checklist

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• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
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Breaking Change

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• No, this is not a breaking change.