Remove duplicate logging issue. (firebase#279)

Some combinations of iOS version, simulator/device, and Xcode version
result in duplicate logs. This should remove duplicate logs while
ensuring that all Firebase related logs are still shown.

Author: ryanwilson

Firebase/Core/FIRLogger.m

@@ -77,8 +77,22 @@
 
 void FIRLoggerInitializeASL() {
   dispatch_once(&sFIRLoggerOnceToken, ^{
+    NSInteger majorOSVersion = [[FIRAppEnvironmentUtil systemVersion] integerValue];
+    uint32_t aslOptions = ASL_OPT_STDERR;
+#if TARGET_OS_SIMULATOR
+    // The iOS 11 simulator doesn't need the ASL_OPT_STDERR flag.
+    if (majorOSVersion >= 11) {
+      aslOptions = 0;
+    }
+#else
+    // Devices running iOS 10 or higher don't need the ASL_OPT_STDERR flag.
+    if (majorOSVersion >= 10) {
+      aslOptions = 0;
+    }
+#endif  // TARGET_OS_SIMULATOR
+
     // Initialize the ASL client handle.
-    sFIRLoggerClient = asl_open(NULL, kFIRLoggerASLClientFacilityName, ASL_OPT_STDERR);
+    sFIRLoggerClient = asl_open(NULL, kFIRLoggerASLClientFacilityName, aslOptions);
 
     // Set the filter used by system/device log. Initialize in default mode.
     asl_set_filter(sFIRLoggerClient, ASL_FILTER_MASK_UPTO(ASL_LEVEL_NOTICE));
@@ -104,11 +118,15 @@ void FIRLoggerInitializeASL() {
       sFIRLoggerDebugMode = NO;
     }
 
-    // Need to call asl_add_output_file so that the logs can appear in Xcode's console view. Set
-    // the ASL filter mask for this output file up to debug level so that all messages are
-    // viewable in the console.
-    asl_add_output_file(sFIRLoggerClient, STDERR_FILENO, kFIRLoggerCustomASLMessageFormat,
-                        ASL_TIME_FMT_LCL, ASL_FILTER_MASK_UPTO(ASL_LEVEL_DEBUG), ASL_ENCODE_SAFE);
+#if TARGET_OS_SIMULATOR
+    // Need to call asl_add_output_file so that the logs can appear in Xcode's console view when
+    // running iOS 7. Set the ASL filter mask for this output file up to debug level so that all
+    // messages are viewable in the console.
+    if (majorOSVersion == 7) {
+      asl_add_output_file(sFIRLoggerClient, STDERR_FILENO, kFIRLoggerCustomASLMessageFormat,
+                          ASL_TIME_FMT_LCL, ASL_FILTER_MASK_UPTO(ASL_LEVEL_DEBUG), ASL_ENCODE_SAFE);
+    }
+#endif  // TARGET_OS_SIMULATOR
 
     sFIRClientQueue = dispatch_queue_create("FIRLoggingClientQueue", DISPATCH_QUEUE_SERIAL);
     dispatch_set_target_queue(sFIRClientQueue,