Enable `intro_flutter_gpu` on master CI

[domesticmouse]

Enabling intro_flutter_gpu with tests skipped, to make sure the code stays sane w.r.t compilation and dart format.

Pre-launch Checklist

• [x] I read the Effective Dart: Style recently, and have followed its advice.
• [x] I signed the CLA.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] All existing and new tests are passing.

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