[macOS] Refactor macOS build/codesize analysis (#137164)

Extracts out _shouldWriteCodeSizeAnalysis and _writeCodeSizeAnalysis from the main buildMacOS function.

No tets changes sine this is a simple restructuring for readability and does not change any tool logic.

packages/flutter_tools/lib/src/macos/build_macos.dart

@@ -140,7 +140,20 @@ Future<void> buildMacOS({
   if (result != 0) {
     throwToolExit('Build process failed');
   }
-  if (buildInfo.codeSizeDirectory != null && sizeAnalyzer != null) {
+  await _writeCodeSizeAnalysis(buildInfo, sizeAnalyzer);
+  globals.flutterUsage.sendTiming('build', 'xcode-macos', Duration(milliseconds: sw.elapsedMilliseconds));
+}
+
+/// Performs a size analysis of the AOT snapshot and writes to an analysis file, if configured.
+///
+/// Size analysis will be run for release builds where the --analyze-size
+/// option has been specified. By default, size analysis JSON output is written
+/// to ~/.flutter-devtools/macos-code-size-analysis_NN.json.
+Future<void> _writeCodeSizeAnalysis(BuildInfo buildInfo, SizeAnalyzer? sizeAnalyzer) async {
+  // Bail out if the size analysis option was not specified.
+  if (buildInfo.codeSizeDirectory == null || sizeAnalyzer == null) {
+    return;
+  }
   final String arch = DarwinArch.x86_64.name;
   final File aotSnapshot = globals.fs.directory(buildInfo.codeSizeDirectory)
     .childFile('snapshot.$arch.json');
@@ -180,6 +193,4 @@ Future<void> buildMacOS({
     '\nTo analyze your app size in Dart DevTools, run the following command:\n'
     'dart devtools --appSizeBase=$relativeAppSizePath'
   );
-  }
-  globals.flutterUsage.sendTiming('build', 'xcode-macos', Duration(milliseconds: sw.elapsedMilliseconds));
 }