_super = function () {
      throw new Error(name + ' is not a function');
    }
### .overwriteMethod(ctx, name, fn)

Overwites an already existing method and provides
access to previous function. Must return function
to be used for name.

    utils.overwriteMethod(chai.Assertion.prototype, 'equal', function (_super) {
      return function (str) {
        var obj = utils.flag(this, 'object');
        if (obj instanceof Foo) {
          new chai.Assertion(obj.value).to.equal(str);
        } else {
          _super.apply(this, arguments);
        }
      }
    });

Can also be accessed directly from `chai.Assertion`.

    chai.Assertion.overwriteMethod('foo', fn);

Then can be used as any other assertion.

    expect(myFoo).to.equal('bar');

@param {Object} ctx object whose method is to be overwritten
@param {String} name of method to overwrite
@param {Function} method function that returns a function to be used for name
@namespace Utils
@name overwriteMethod
@api public