How to handle warnings for proprietary/custom properties of built-in objects in TypeScript

I am using Personas which relies on the proprietary property navigator.id. Since this property is not standard, the TypeScript compiler generates the following warning:

$ tsc home.ts --out my_ts_generated_code.js
/Users/..../home.ts(27,18): The property 'id' does not exist on value of type 'Navigator'

But the .js file is successfully generated and runs on the FF15 browser without any warning/error message.
I also include a polyfill for navigator.id, as instructed by the documentation, so navigator.id will definitely by available in every browser.

Could someone suggest me how to deal with this warning?

index.html

<!-- some HTML omit above -->
<script src="https://login.persona.org/include.js"></script>
<script src="my_ts_generated_code.js"></script>
<button class="btn" id="signin">Sign in</button>
<button class="btn" id="signout">Sign out</button>
<!-- some HTML omit below -->

home.ts

declare var $;

class Student {
    fullname : string;
    constructor(public firstname, public middleinitial, public lastname) {
        this.fullname = firstname + " " + middleinitial + " " + lastname;
    }
}

interface Person {
    firstname: string;
    lastname: string;
}

function greeter(person : Person) {
    return "Hello, " + person.firstname + " " + person.lastname;
}

var user = new Student("Jane", "M.", "User");

$(function() {
    $('#signin').on('click', function(e) {
        e.preventDefault();
        navigator.id.request();
    });

    $('#signout').on('click', function(e) {
        e.preventDefault();
        navigator.id.logout();
    });
    //document.body.innerHTML = greeter(user);
});