[firestore_serialize] Not compatible with analyzer 0.40.0

[ciriousjoker]

For some reason, to get firestore_serialize to build, right now I have to override to analyzer 0.39.14 since 0.39.15 breaks.0.39.14 doesn't seem to be compatible with flutter >1.18, so right now I'm stuck an old flutter version.

[ciriousjoker]

@lukas-h

• I've forked this repo and fixed this issue locally. My fork now has all dependencies at their latest versions.
• Also, I updated the syntax to the latest cloud_firestore (0.14), which uses .data()["field"] instead of .data["field"]

In fixing the repo, I broke the test case though. It does compile perfectly using this command:
flutter pub run build_runner build --delete-conflicting-outputs

This is the error message when I'm trying to run the test:

searchable text

``` Expected: empty Actual: Set:[ 'firestore_serializable/lib/src/deserialize_helper.dart', 'firestore_serializable/lib/src/helper.dart', 'firestore_serializable/lib/src/serialize_helper.dart', 'firestore_serializable/lib/src/utils.dart' ]

package:test_api expect
expectBuildCleanImpl
package:build_verify/src/impl.dart:26
expectBuildClean
package:build_verify/build_verify.dart:23
main.
test/ensure_build_test.dart:8

</details>

[ciriousjoker]

@lukas-h Can you merge the PR please?

#42