I only knew the command line option which no longer worked. Once I found the project.json

    "buildOptions": {
        "allowUnsafe": true
    }

I was able to fake something for dotnet migrate and find the needed setting:

    <Project Sdk="Microsoft.NET.Sdk">
        <PropertyGroup>
            <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
        </PropertyGroup>
    </Project>