Cleanup: Made a macro to assert ARC is enabled. (#12701)

Cleanup: Made a macro to assert ARC is enabled (and its inverse).

shell/platform/darwin/common/framework/Headers/FlutterMacros.h

@@ -37,4 +37,12 @@
  */
 #define FLUTTER_UNAVAILABLE(msg) __attribute__((__unavailable__(msg)))
 
+#if __has_feature(objc_arc)
+#define FLUTTER_ASSERT_ARC
+#define FLUTTER_ASSERT_NOT_ARC #error ARC must be disabled!
+#else
+#define FLUTTER_ASSERT_ARC #error ARC must be enabled!
+#define FLUTTER_ASSERT_NOT_ARC
+#endif
+
 #endif  // FLUTTER_FLUTTERMACROS_H_

shell/platform/darwin/ios/framework/Source/FlutterBinaryMessengerRelayTest.mm

@@ -4,15 +4,10 @@
 
 #import <OCMock/OCMock.h>
 #import <XCTest/XCTest.h>
+#include "flutter/shell/platform/darwin/common/framework/Headers/FlutterMacros.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterBinaryMessengerRelay.h"
 
-#ifndef __has_feature
-#define __has_feature(x) 0 /* for non-clang compilers */
-#endif
-
-#if !__has_feature(objc_arc)
-#error ARC must be enabled!
-#endif
+FLUTTER_ASSERT_ARC
 
 @interface FlutterBinaryMessengerRelayTest : XCTestCase
 @end

shell/platform/darwin/ios/framework/Source/FlutterEngineTest.mm

@@ -4,15 +4,10 @@
 
 #import <OCMock/OCMock.h>
 #import <XCTest/XCTest.h>
+#include "flutter/shell/platform/darwin/common/framework/Headers/FlutterMacros.h"
 #import "flutter/shell/platform/darwin/ios/framework/Headers/FlutterEngine.h"
 
-#ifndef __has_feature
-#define __has_feature(x) 0 /* for non-clang compilers */
-#endif
-
-#if !__has_feature(objc_arc)
-#error ARC must be enabled!
-#endif
+FLUTTER_ASSERT_ARC
 
 @interface FlutteEngineTest : XCTestCase
 @end

shell/platform/darwin/ios/framework/Source/FlutterPluginAppLifeCycleDelegateTest.m

@@ -4,15 +4,10 @@
 
 #import <OCMock/OCMock.h>
 #import <XCTest/XCTest.h>
+#include "flutter/shell/platform/darwin/common/framework/Headers/FlutterMacros.h"
 #import "flutter/shell/platform/darwin/ios/framework/Headers/FlutterPluginAppLifeCycleDelegate.h"
 
-#ifndef __has_feature
-#define __has_feature(x) 0 /* for non-clang compilers */
-#endif
-
-#if !__has_feature(objc_arc)
-#error ARC must be enabled!
-#endif
+FLUTTER_ASSERT_ARC
 
 @interface FlutterPluginAppLifeCycleDelegateTest : XCTestCase
 

shell/platform/darwin/ios/framework/Source/FlutterViewControllerTest.m

@@ -4,13 +4,12 @@
 
 #import <OCMock/OCMock.h>
 #import <XCTest/XCTest.h>
+#include "flutter/shell/platform/darwin/common/framework/Headers/FlutterMacros.h"
 #import "flutter/shell/platform/darwin/ios/framework/Headers/FlutterViewController.h"
 
 #include "FlutterBinaryMessenger.h"
 
-#if !__has_feature(objc_arc)
-#error ARC must be enabled!
-#endif
+FLUTTER_ASSERT_ARC
 
 extern NSNotificationName const FlutterViewControllerWillDealloc;