Revert changes introduced in #546 to fix build breakage. (#562)

Author: ryanwilson

Firebase/Messaging/FIRMessaging.m

@@ -384,6 +384,7 @@ - (void)handleIncomingLinkIfNeededFromMessage:(NSDictionary *)message {
   id<UIApplicationDelegate> appDelegate = application.delegate;
   SEL continueUserActivitySelector =
       @selector(application:continueUserActivity:restorationHandler:);
+  SEL openURLWithOptionsSelector = @selector(application:openURL:options:);
   SEL openURLWithSourceApplicationSelector =
       @selector(application:openURL:sourceApplication:annotation:);
   SEL handleOpenURLSelector = @selector(application:handleOpenURL:);
@@ -402,7 +403,7 @@ - (void)handleIncomingLinkIfNeededFromMessage:(NSDictionary *)message {
       // Do nothing, as we don't support the app calling this block
     }];
 
-  } else if (@available(iOS 9.0, *)) {
+  } else if ([appDelegate respondsToSelector:openURLWithOptionsSelector]) {
     [appDelegate application:application openURL:url options:@{}];
 
   // Similarly, |application:openURL:sourceApplication:annotation:| will also always be called, due
@@ -726,7 +727,7 @@ - (void)setAPNSToken:(NSData *)apnsToken error:(NSError *)error {
 
 - (void)receiver:(FIRMessagingReceiver *)receiver
       receivedRemoteMessage:(FIRMessagingRemoteMessage *)remoteMessage {
-  if (@available(iOS 10.0, *)) {
+  if ([self.delegate respondsToSelector:@selector(messaging:didReceiveMessage:)]) {
     [self.delegate messaging:self didReceiveMessage:remoteMessage];
   } else if ([self.delegate respondsToSelector:@selector(applicationReceivedRemoteMessage:)]) {
 #pragma clang diagnostic push

Firebase/Messaging/FIRMessagingContextManagerService.m

@@ -143,7 +143,7 @@ + (void)scheduleLocalNotificationForMessage:(NSDictionary *)message
   }
   if ([apsDictionary[kFIRMessagingContextManagerTitleKey] length]) {
     // |alertTitle| is iOS 8.2+, so check if we can set it
-    if (@available(iOS 8.2, *)) {
+      if ([notification respondsToSelector:@selector(setAlertTitle:)]) {
       notification.alertTitle = apsDictionary[kFIRMessagingContextManagerTitleKey];
     }
   }

Firebase/Messaging/FIRMessagingRmq2PersistentStore.m

@@ -104,13 +104,9 @@
 
 // Utility to create an NSString from a sqlite3 result code
 NSString * _Nonnull FIRMessagingStringFromSQLiteResult(int result) {
-  const char *errorStr;
-  if (@available(iOS 8.2, *)) {
-    errorStr = sqlite3_errstr(result);
-  } else {
-    errorStr = "pre iOS 8.2";
-  }
-  return [NSString stringWithFormat:@"%d - %s", result, errorStr];
+  const char *errorStr = sqlite3_errstr(result);
+  NSString *errorString = [NSString stringWithFormat:@"%d - %s", result, errorStr];
+  return errorString;
 }
 
 @interface FIRMessagingRmq2PersistentStore () {