Unsafe code compilation error in .NET Core even after setting allowunsafe flag to true in project.json

I am using some unsafe code in my .NET Core app. For that, I had made this change in the project.json file:

"compilationOptions": {
    "allowUnsafe": true,
}

However, I still get the error CS0227: Unsafe code may only appear if compiling with /unsafe.

I had already gone through this:

• Unsafe code won't compile on Visual Studio 2015
• How to call unsafe code from ASP.NET xproj

NOTE:
This answer is based on earlier .Net Core/Standard project formats written in json. For the new formatted csproj see Ray's answer.

Change compilationOptions to buildOptions:

"buildOptions": {
    "allowUnsafe": true
}