Check that -XDshould-stop.ifError=FLOW is set when running Error Prone

[copybara-service]

Check that -XDshould-stop.ifError=FLOW is set when running Error Prone

This is necessary to ensure javac compiles the 'flow' phase for subsequent compilation units after Error Prone reports diagnostics. The 'flow' phase is necessary among other things to set EFFECTIVELY_FINAL. Configuring a later stop policy also causes more diagnostics to be shown at once for failing builds when using -XDcompilePolicy=simple, which is helpful.

#4595

RELNOTES=The javac flag -XDshould-stop.ifError=FLOW is now required when running Error Prone, see #4595

[tbroyer]

@cushon Do you know off-hand if versions of ErrorProne before 2.11 (those versions compatible with JDK 8) would benefit from the JDK 8 -XDshouldStopPolicyIfError=FLOW?
I'm in the process of adding -XDshould-stop.ifError=FLOW to the gradle-errorprone-plugin and noticed it appeared in JDK 9, so wondered if I should add both variants for JDK 8 (that I currently still support, with older Error Prone versions) and JDK 9+, or if the JDK 9+ variant would be enough.

Fwiw, I won't write a test for that; I apparently can just pass both options and javac doesn't care and just ignores the unknown one, but if you tell me it's useless to pass the JDK 8 variant then I'll just pass the JDK 9+ variant.

[cushon]

@cushon Do you know off-hand if versions of ErrorProne before 2.11 (those versions compatible with JDK 8) would benefit from the JDK 8 -XDshouldStopPolicyIfError=FLOW?

Yes, I believe the same behaviour exists in JDK 8 and it would benefit from the equivalent flag. (The flag got renamed in JDK 9 by openjdk/jdk@29aa24a)

I also left a related note in #4595 (comment), I'm not completely sure what form of the flag we want to recommend when this change is released.

[tbroyer]

Oh, BTW, I noticed 2 uses of the JDK 8 name in the codebase (in tests): https://github.com/search?q=repo%3Agoogle%2Ferror-prone%20shouldStopPolicyIfError&type=code

[cushon]

Oh, BTW, I noticed 2 uses of the JDK 8 name in the codebase (in tests): https://github.com/search?q=repo%3Agoogle%2Ferror-prone%20shouldStopPolicyIfError&type=code

Thanks! I will clean those up

[tbroyer]

May I ask you for a quick review? tbroyer/gradle-errorprone-plugin#109
In the end, I added a test inspired by the ones added here.

I might revisit this later to conditionally pass -XDshouldStopPolicyIfError=FLOW for JDK 8, --should-stop:ifError=FLOW for JDK 9 and 10, and --should-stop=ifError=FLOW for JDK 11+
Let me know if you think I should do that from the start. TIA