@SuppressWarnings("unchecked")
    public B addMixin(Shape shape) {
        if (mixins == null) {
            mixins = new LinkedHashMap<>();
        }

        mixins.put(shape.getId(), shape);
        return (B) this;
    }
Adds a mixin to the shape.

@param shape Mixin to add.
@return Returns the builder.
smithy-model/src/main/java/software/amazon/smithy/model/shapes/AbstractShapeBuilder.java