Trigger paper-input validations

I have a modal which consists of several parts (it's basically a multi-step form). There's a **Next** button to advance. However, I don't want users to advance without correctly filling all fields in whatever step they are. Each `paper-input` in the form has a validation. My idea is to check the validity of all paper inputs once the user clicks on `Next`. Is it possible to somehow trigger those validations?

No file or test is named. Start by locating the paper-input validation implementation and the multi-step form's Next-button flow described in the issue, then determine the supported behavior for triggering validation; done should mean advancement is blocked when fields in the current step are invalid.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.