Add support for headless login to firebase-auth.

firebase-auth.html

@@ -187,13 +187,17 @@
      * provider-specific login parameters can be specified via attribute (JSON)/property,
      * or via the `params` argument to the `login` function.
      *
+     * If your `provider` is `custom` you must pass a Firebase Auth token as
+     * `params.token`. You can also optionally pass an auth token as `params.token` for
+     * providers `facebook`, `google`, `github` and `twitter` to login headlessly.
+     *
      * If the login is successful, the `login` event is fired, with `e.detail.user`
      * containing the authenticated user object from Firebase.
      *
      * If login fails, the `error` event is fired, with `e.detail` containing error
      * information supplied from Firebase.
      *
-     * If the browswer supports `navigator.onLine` network status reporting and the
+     * If the browser supports `navigator.onLine` network status reporting and the
      * network is currently offline, the login attempt will be queued until the network
      * is restored.
      *
@@ -221,7 +225,9 @@
           case 'google':
           case 'github':
           case 'twitter':
-            if (this.redirect) {
+            if (params.token) {
+              this.ref.authWithOAuthToken(this.provider, params.token, this._loginHandler.bind(this), params);
+            } else if (this.redirect) {
               this.ref.authWithOAuthRedirect(this.provider, this._loginHandler.bind(this), params);
             } else {
               this.ref.authWithOAuthPopup(this.provider, this._loginHandler.bind(this), params);