fix(auth): fix typings and externs for PhoneAuthProvider.credential

Author: jsayol

externs/firebase-auth-externs.js

@@ -1811,7 +1811,7 @@ firebase.auth.PhoneAuthProvider.PROVIDER_ID;
  *     {@link firebase.auth.PhoneAuthProvider#verifyPhoneNumber}.
  * @param {string} verificationCode The verification code sent to the user's
  *     mobile device.
- * @return {!firebase.Promise<!firebase.auth.AuthCredential>}
+ * @return {!firebase.auth.AuthCredential} The auth provider credential.
  */
 firebase.auth.PhoneAuthProvider.credential =
     function(verificationId, verificationCode) {};

typings/app.d.ts

@@ -196,7 +196,7 @@ declare namespace firebase.auth {
 
   class PhoneAuthProvider extends PhoneAuthProvider_Instance {
     static PROVIDER_ID : string ;
-    static credential (verificationId : string , verificationCode : string ) : firebase.Promise < any > ;
+    static credential (verificationId : string , verificationCode : string ) : firebase.auth.AuthCredential ;
   }
   class PhoneAuthProvider_Instance implements firebase.auth.AuthProvider {
     constructor (auth ? : firebase.auth.Auth | null ) ;

typings/firebase.d.ts

@@ -196,7 +196,7 @@ declare namespace firebase.auth {
 
   class PhoneAuthProvider extends PhoneAuthProvider_Instance {
     static PROVIDER_ID : string ;
-    static credential (verificationId : string , verificationCode : string ) : firebase.Promise < any > ;
+    static credential (verificationId : string , verificationCode : string ) : firebase.auth.AuthCredential ;
   }
   class PhoneAuthProvider_Instance implements firebase.auth.AuthProvider {
     constructor (auth ? : firebase.auth.Auth | null ) ;