Use emulator to run tests

[cachapa]

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Currently the Firedart automated tests work directly against a real Firebase project.

This has a number of problems:

• It's slow
• Is resource intensive
• Depends on the state of the network
• Depends on Firebase itself
• Risks exposing credentials

But the biggest problem is that the tests cannot be run automatically on PRs in a safe manner.

Firedart now supports Firebase emulator, so it should be possible to use that for testing.