Remove usages of goog.isFunction (#3205)

* Update args.js

* Update auth.js

Author: kjin

packages/auth/src/args.js

@@ -307,7 +307,7 @@ fireauth.args.func = function(opt_name, opt_optional) {
     name: opt_name || '',
     typeLabel: 'a function',
     optional: !!opt_optional,
-    validator: goog.isFunction
+    validator: x => typeof x === 'function'
   };
 };
 
@@ -564,7 +564,7 @@ fireauth.args.phoneInfoOptions = function(name, optional) {
 fireauth.args.validateMultiFactorSession_ = function(session, type) {
   return goog.isObject(session) && typeof session.type === 'string' &&
       session.type === type &&
-      goog.isFunction(session.getRawSession);
+      typeof session.getRawSession === 'function';
 };
 
 
@@ -613,7 +613,7 @@ fireauth.args.applicationVerifier = function(opt_optional) {
         function(applicationVerifier) {
           return !!(applicationVerifier &&
                     typeof applicationVerifier.type === 'string' &&
-                    goog.isFunction(applicationVerifier.verify));
+                    typeof applicationVerifier.verify === 'function');
         })
   });
 };

packages/auth/src/auth.js

@@ -1412,9 +1412,9 @@ fireauth.Auth.prototype.onIdTokenChanged = function(
     // and different promises library and this leads to calls resolutions order
     // being different from the promises registration order.
     Promise.resolve().then(function() {
-      if (goog.isFunction(nextOrObserver)) {
+      if (typeof nextOrObserver === 'function') {
         nextOrObserver(self.currentUser_());
-      } else if (goog.isFunction(nextOrObserver['next'])) {
+      } else if (typeof nextOrObserver['next'] === 'function') {
         nextOrObserver['next'](self.currentUser_());
       }
     });
@@ -1458,9 +1458,9 @@ fireauth.Auth.prototype.onAuthStateChanged = function(
       // it has the correct UID before triggering the user state change
       // listeners.
       self.userStateChangeUid_ = self.getUid();
-      if (goog.isFunction(nextOrObserver)) {
+      if (typeof nextOrObserver === 'function') {
         nextOrObserver(self.currentUser_());
-      } else if (goog.isFunction(nextOrObserver['next'])) {
+      } else if (typeof nextOrObserver['next'] === 'function') {
         nextOrObserver['next'](self.currentUser_());
       }
     });