Use #if TARGET_OS_IOS instead of #ifdef TARGET_OS_IOS.

runtime/vm/os_macos.cc

Index: runtime/vm/os_macos.cc
diff --git a/runtime/vm/os_macos.cc b/runtime/vm/os_macos.cc
index a447e4187e67125369c3c91b089011fef45fe7f9..aed5d33529bbc28a7e0268d76fc87dd3d80853e6 100644
--- a/runtime/vm/os_macos.cc
+++ b/runtime/vm/os_macos.cc
@@ -15,7 +15,7 @@
 #include <sys/time.h>  // NOLINT
 #include <sys/resource.h>  // NOLINT
 #include <unistd.h>  // NOLINT
-#if defined(TARGET_OS_IOS)
+#if TARGET_OS_IOS
 #include <sys/sysctl.h>
 #endif
 
@@ -86,7 +86,7 @@ int64_t OS::GetCurrentTimeMicros() {
 
 
 int64_t OS::GetCurrentTraceMicros() {
-#if defined(TARGET_OS_IOS)
+#if TARGET_OS_IOS
   // On iOS mach_absolute_time stops while the device is sleeping. Instead use
   // now - KERN_BOOTTIME to get a time difference that is not impacted by clock
   // changes. KERN_BOOTTIME will be updated by the system whenever the system
@@ -118,7 +118,7 @@ int64_t OS::GetCurrentTraceMicros() {
   result *= timebase_info.numer;
   result /= timebase_info.denom;
   return result;
-#endif  // defined(TARGET_OS_IOS)
+#endif  // TARGET_OS_IOS
 }