Fixes a flake in reload shaders tests (flutter#184268)

fixes flutter#184265

This fixes an oddity related to mmap files. A delay had to be introduced
in the test because of oddities with the filesystem. I tried a few
tricks to get it to work without the delay but this is the only way I
could get it to work.

testing: has existing test, removes a flake

I tested it manually locally:
```
for i in {1..30}; do echo "Running iteration: $i"; ./testing/run_tests.py --type=dart --dart-filter=shader_reload_test.dart --variant host_debug_unopt_arm64; done
```

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we expect every widget to implement].
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above.
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[test-exempt].
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Fixes] where supported.
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Author: gaaclarke

engine/src/flutter/testing/dart/vm_service/shader_reload_test.dart

@@ -76,15 +76,15 @@ void main() {
     final Uint8List sourceA = File(shaderSrcA).readAsBytesSync();
     final Uint8List sourceB = File(shaderSrcB).readAsBytesSync();
 
-    fileC.writeAsBytesSync(sourceA, flush: true);
+    await _deleteAndWrite(fileC, sourceA);
 
     try {
       final FragmentProgram program = await FragmentProgram.fromAsset(testAssetName);
       final FragmentShader shader = program.fragmentShader();
       final UniformFloatSlot slot = shader.getUniformFloat('iVec2Uniform');
       expect(slot.shaderIndex, 1);
 
-      fileC.writeAsBytesSync(sourceB, flush: true);
+      await _deleteAndWrite(fileC, sourceB);
       await _performReload(testAssetName);
       expect(slot.shaderIndex, 5);
     } finally {
@@ -110,15 +110,15 @@ void main() {
     final Uint8List sourceA = File(shaderSrcA).readAsBytesSync();
     final Uint8List sourceB = File(shaderSrcB).readAsBytesSync();
 
-    fileC.writeAsBytesSync(sourceA, flush: true);
+    await _deleteAndWrite(fileC, sourceA);
 
     try {
       final FragmentProgram program = await FragmentProgram.fromAsset(testAssetName);
       final FragmentShader shader = program.fragmentShader();
       final UniformFloatSlot slot = shader.getUniformFloat('iMat2Uniform', 3);
       expect(slot.shaderIndex, 6);
 
-      fileC.writeAsBytesSync(sourceB, flush: true);
+      await _deleteAndWrite(fileC, sourceB);
       await _performReload(testAssetName);
       expect(slot.shaderIndex, 7);
       // Make sure there is no crash here.
@@ -146,15 +146,15 @@ void main() {
     final Uint8List sourceA = File(shaderSrcA).readAsBytesSync();
     final Uint8List sourceB = File(shaderSrcB).readAsBytesSync();
 
-    fileC.writeAsBytesSync(sourceA, flush: true);
+    await _deleteAndWrite(fileC, sourceA);
 
     try {
       final FragmentProgram program = await FragmentProgram.fromAsset(testAssetName);
       final FragmentShader shader = program.fragmentShader();
       final UniformFloatSlot slotA = shader.getUniformFloat('iFloatUniform');
       slotA.set(1.0);
 
-      fileC.writeAsBytesSync(sourceB, flush: true);
+      await _deleteAndWrite(fileC, sourceB);
       await _performReload(testAssetName);
 
       final UniformFloatSlot slotB = shader.getUniformFloat('iFloatUniformRenamed');
@@ -184,15 +184,15 @@ void main() {
     final Uint8List sourceA = File(shaderSrcA).readAsBytesSync();
     final Uint8List sourceB = File(shaderSrcB).readAsBytesSync();
 
-    fileC.writeAsBytesSync(sourceA, flush: true);
+    await _deleteAndWrite(fileC, sourceA);
 
     try {
       final FragmentProgram program = await FragmentProgram.fromAsset(testAssetName);
       final FragmentShader shader = program.fragmentShader();
       final ImageSamplerSlot slot = shader.getImageSampler('tex_a');
       expect(slot.shaderIndex, 0);
 
-      fileC.writeAsBytesSync(sourceB, flush: true);
+      await _deleteAndWrite(fileC, sourceB);
       await _performReload(testAssetName);
       expect(slot.shaderIndex, 1);
     } finally {
@@ -204,3 +204,18 @@ void main() {
 }
 
 void _use(Shader shader) {}
+
+/// Delete the file first, if it exists, to ensure the engine doesn't attempt
+/// to read a memory-mapped file that is actively being truncated and written to.
+Future<void> _deleteAndWrite(File file, Uint8List bytes) async {
+  if (file.existsSync()) {
+    file.deleteSync();
+  }
+  file.writeAsBytesSync(bytes, flush: true);
+  // Allow time for the macOS file system metadata caching to settle.
+  // Otherwise, the engine's subsequent `fstat` may read a `0` st_size
+  // or `openat` may fail entirely due to APFS asynchronous directory updates.
+  //
+  // renameSync() was tried but it was not sufficient to fix the issue.
+  await Future<void>.delayed(const Duration(milliseconds: 100));
+}