function getAssert(reject) {
  return function assert(condition, message) {
    if (condition) {
      return;
    }
    reject(new Error(message));
  };
}
Using expect.js in an async test function doesn't give reliable stack traces
and failures result in a message about the done callback not being called.

@param {function(Error):void} reject The test callback.
@return {function(boolean, string):void} The assert function.