* Enable additional Error Prone checks & fix violations
Some of them also enforce additional Google Java Format requirements which
are not handled by google-java-format, such as disallowing wildcard imports.
Not all experimental checks have been listed because some are not applicable,
such as Dependency Injection framework checks, or checks related to Guava's
immutable collections (since Gson's main code does not have a dependency on
Guava).
Other checks have been omitted because they are probably not relevant
(this was a subjective choice), or would require larger refactoring or
would flag issues with the public API, which cannot be changed easily.
* Address review feedback
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Co-authored-by: Éamonn McManus <emcmanus@google.com>
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