Fluent interfaces are easily supported in C# by using `return` `this`.

In Linq, extension methods are used to accomplish the same thing.  Extension methods over `IQueryable` or `IEnumerable` are easily chained together.

Cascades, as described by the page at the link you provided, seem to be merely a subtle refinement of an ordinary Fluent Interface where `return` `this` is not required to make it work, as all method calls beginning with `..` refer to the first returned result:


    query('#my-form').query('button')
        ..classes.add('toggle')
        ..text = 'Click Me!'
        ..labels.add(toggleLabel);

Does C# support this?  No, not exactly.  But you can still write the following equivalent code:

    var result = query('#my-form').query('button')
    result.classes.add('toggle')
    result.text = 'Click Me!'
    result.labels.add(toggleLabel);

and it is only slightly more verbose (and arguably easier to read).

As to the question, "Were they ever considered for C#," you would have to ask Anders Hejlsberg or Eric Lippert about that.  My guess is that they view it as syntactic sugar, which in fact it is. 

Although Dart claims that this syntax means you don't have to choose between chaining and having a meaningful return value, I don't see the point, as the syntax doesn't provide a place for the return value to go.