Adding virtual to a member

While this change would often work without breaking too many scenarios because C# compiler tends to emit callvirt IL instructions to call non-virtual methods (callvirt performs a null check, while a normal call won't), we can't rely on it. C# is not the only language we target and the C# compiler increasingly tries to optimize callvirt to a normal call whenever the target method is non-virtual and the this is provably not null (such as a method accessed through the ?. null propagation operator). Making a method virtual would mean that consumer code would often end up calling it non-virtually.