Improve FirebaseMessaging remote notification proxy source code (#13818)

FirebaseMessaging/Sources/FIRMessagingRemoteNotificationsProxy.m

@@ -300,10 +300,10 @@ static NSString *kUserNotificationDidReceiveResponseSelectorString =
     IMP originalMethodImplementation =
         method_setImplementation(originalMethod, swizzledImplementation);
 
-    IMP nonexistantMethodImplementation = [self nonExistantMethodImplementationForClass:klass];
+    IMP nonexistentMethodImplementation = [self nonExistentMethodImplementationForClass:klass];
 
     if (originalMethodImplementation &&
-        originalMethodImplementation != nonexistantMethodImplementation &&
+        originalMethodImplementation != nonexistentMethodImplementation &&
         originalMethodImplementation != swizzledImplementation) {
       [self saveOriginalImplementation:originalMethodImplementation forSelector:originalSelector];
     }
@@ -344,8 +344,8 @@ static NSString *kUserNotificationDidReceiveResponseSelectorString =
     // behavior as if the method was not implemented.
     // See: http://stackoverflow.com/a/8276527/9849
 
-    IMP nonExistantMethodImplementation = [self nonExistantMethodImplementationForClass:klass];
-    method_setImplementation(swizzledMethod, nonExistantMethodImplementation);
+    IMP nonExistentMethodImplementation = [self nonExistentMethodImplementationForClass:klass];
+    method_setImplementation(swizzledMethod, nonExistentMethodImplementation);
   }
 }
 
@@ -354,10 +354,10 @@ static NSString *kUserNotificationDidReceiveResponseSelectorString =
 // This is useful to generate from a stable, "known missing" selector, as the IMP can be compared
 // in case we are setting an implementation for a class that was previously "unswizzled" into a
 // non-existent implementation.
-- (IMP)nonExistantMethodImplementationForClass:(Class)klass {
-  SEL nonExistantSelector = NSSelectorFromString(@"aNonExistantMethod");
-  IMP nonExistantMethodImplementation = class_getMethodImplementation(klass, nonExistantSelector);
-  return nonExistantMethodImplementation;
+- (IMP)nonExistentMethodImplementationForClass:(Class)klass {
+  SEL nonExistentSelector = NSSelectorFromString(@"aNonExistentMethod");
+  IMP nonExistentMethodImplementation = class_getMethodImplementation(klass, nonExistentSelector);
+  return nonExistentMethodImplementation;
 }
 
 // A safe, non-leaky way return a property object by its name