Invalid automatic interface implementation in abstract class

[sharwell]

Sharpen generates incorrect code for the following input:

interface Foo {
    /** @sharpen.indexer */
    int get(int index);
}

abstract class AbstractFoo implements Foo {
}

Here is the current output.

internal interface IFoo
{
    int this[int index]
    {
        get;
    }
}

internal abstract class AbstractFoo : IFoo
{
    public abstract int Get(int arg1);
}