fn = function() {
    'use strict';
    var args = Array.prototype.slice.call(arguments);
    args.splice(0, 0, opt_functionName || '[anonymous mocked function]');
    return fn.$mockMethod.apply(fn, args);
  }
Class used to mock a function. Useful for mocking closures and anonymous
callbacks etc. Creates a function object that extends goog.testing.Mock.
@param {string=} opt_functionName The optional name of the function to mock.
    Set to '[anonymous mocked function]' if not passed in.
@param {number=} opt_strictness One of goog.testing.Mock.LOOSE or
    goog.testing.Mock.STRICT. The default is STRICT.
@return {!goog.testing.MockInterface} The mocked function.
@suppress {missingProperties} Mocks do not fit in the type system well.