function expectOk(message) {
  mockOkHandler.handleEvent(new ArgumentMatcher(function(arg) {
    return arg.type == EventType.OK && arg.message == message;
  }));
}
Sets up the mock event handler to expect an OK event with the given
message.
@param {string} message Hello world message the OK event is expected to
    carry.
@suppress {missingProperties} suppression added to enable type checking