Make FieldValue a test-only class

[schmidt-sebastian]

With all the prior PRs done, we now no longer need FieldValue. I moved it to a testHelpers for now to keep the diff smaller. The next step would be to remove it altogether and to return Value types from wrap.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (mrschmidt/rewritefieldvalue@666458a). Click here to learn what that means.
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Flag	Coverage Δ	Complexity Δ
#FirebaseFirestore	61.92% <0%> (?)	2225% <0%> (?)
#FirebaseFirestore_Ktx	41.17% <0%> (?)	0% <0%> (?)

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[wilhuff]

LGTM

[wilhuff]

I'm surprised this works. Is the only usage of this in UserDataWriter?

[schmidt-sebastian]

The class is only used in UserDataWriter and ProtoValues. isServerTimestamp() is used in a couple other places.

[google-oss-bot]

@schmidt-sebastian: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
check-changed	739e646	link	/test check-changed

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