[macOS] Migrate from assert to FML_DCHECK

Eliminates raw C asserts in the macOS embedder, replacing them with
FML_DCHECK for consistency with the rest of the codebase.

No semantic change so no changes to tests.

Author: cbracken

shell/platform/darwin/macos/framework/Source/FlutterSurfaceManager.mm

@@ -3,11 +3,14 @@
 // found in the LICENSE file.
 
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterSurfaceManager.h"
-#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterSurface.h"
 
 #import <Metal/Metal.h>
+
 #include <algorithm>
 
+#include "flutter/fml/logging.h"
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterSurface.h"
+
 @implementation FlutterSurfacePresentInfo
 @end
 
@@ -75,7 +78,7 @@ - (FlutterSurface*)surfaceForSize:(CGSize)size {
 }
 
 - (void)commit:(NSArray<FlutterSurfacePresentInfo*>*)surfaces {
-  assert([NSThread isMainThread]);
+  FML_DCHECK([NSThread isMainThread]);
 
   // Release all unused back buffer surfaces and replace them with front surfaces.
   [_backBufferCache replaceSurfaces:_frontSurfaces];

shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.mm

@@ -1,11 +1,13 @@
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.h"
-#import "fml/synchronization/waitable_event.h"
 
 #import <QuartzCore/QuartzCore.h>
 
 #include <mutex>
 #include <vector>
 
+#import "flutter/fml/logging.h"
+#import "flutter/fml/synchronization/waitable_event.h"
+
 @interface FlutterThreadSynchronizer () {
   std::mutex _mutex;
   BOOL _shuttingDown;
@@ -23,7 +25,7 @@ @interface FlutterThreadSynchronizer () {
 @implementation FlutterThreadSynchronizer
 
 - (void)drain {
-  assert([NSThread isMainThread]);
+  FML_DCHECK([NSThread isMainThread]);
 
   [CATransaction begin];
   [CATransaction setDisableActions:YES];