Support for init properties in dynamic binder

[HaloFour]

See: dotnet/csharplang#39 (comment)

Seems that the C# runtime binder ignores modreq metadata on members. This enables init properties to be trivially mutated by casting to dynamic:

public class Foo {
    public string Value { get; init; }
}

static class Program {
    static void Main() {
        var foo = new Foo { Value = "Hello" };
        ((dynamic)foo).Value = "World";
        Console.WriteLine(foo.Value); // prints World
    }
}

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[mangod9]

@HaloFour I assume you expect this to not compile in c#, additionally the runtime to also throw?

[HaloFour]

@mangod9

I'd expect it to compile in C# (since it's being cast to dynamic) but to throw at runtime. It probably should be doing this already since the member would have a modreq applied to it and shouldn't be invoked without the binder understanding what that modifier entails.

If you take dynamic out of the equation and attempt to mutate the property it will fail to compile: SharpLab

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[HaloFour]

Also see: dotnet/csharplang#3376 (comment)

@jaredpar