System.Text.Encodings.Web.dll ref api compat issue in preview 8

[aik-jahoda]

There is System.Text.Encodings.Web.dll failure in release/5.0-preview8
The problem is visible here:
https://github.com/dotnet/runtime/pull/39434/checks?check_run_id=877743259

Error message:

MatchingRefApiCompat failed - The reference assembly doesn't match all the APIs in the implementation for 'artifacts\bin\System.Text.Encodings.Web\netcoreapp3.0-Debug\System.Text.Encodings.Web.dll'. To address either fix errors in the reference assembly (referenced as implementation in compat errors for this reverse compat check), add the issues to the baseline file 'src\libraries\System.Text.Encodings.Web\src\MatchingRefApiCompatBaseline.txt' or disable this check by setting RunMatchingRefApiCompat=false in this project.

cc: @Anipik @ericstj @GrabYourPitchforks @layomia @tarekgh

[ghost]

Tagging subscribers to this area: @tarekgh, @krwq
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[ericstj]

Here's the error messages from https://dev.azure.com/dnceng/public/_build/results?buildId=733219&view=logs&j=423ae147-26a3-5a4d-a4d3-4b3fc1dd598e&t=98a4dd5a-baea-5dc4-484b-ccbd315f07c8&l=4551

error : TypesMustExist : Type 'System.Runtime.Intrinsics.Arm.AdvSimd' does not exist in the reference but it does exist in the implementation. 
error : TypesMustExist : Type 'System.Runtime.Intrinsics.Arm.ArmBase' does not exist in the reference but it does exist in the implementation. 

I believe regression was caused by #38707. @eiriktsarpalis

[aik-jahoda]

For the System.Runtime.Intrinsics.Arm.AdvSimd I created an issue there: #39440.

[eiriktsarpalis]

I think this might be caused by the shimmed AdvSimd APIs used for netcoreapp3.0 builds. Something is causing the USE_INTERNAL_ACCESSIBILITY constant not being defined. I'll take a look.

[aik-jahoda]

I expect the fix can be as simple as in: 6be3ce7

The IsPrerelease has changed in this branch: bf35bfd

[eiriktsarpalis]

Tagging @pgovind

[ericstj]

@eiriktsarpalis can this be closed now?