fix(build): preserve flutter-packages on --skip-site-packages incremental builds (#6634)

register_flutter_extensions() unconditionally wiped the permanent
build/flutter-packages directory before repopulating it only when the
temp dir exists. On incremental builds the package hash is unchanged, so
serious_python is invoked with --skip-site-packages and does not copy
Flutter packages to the temp dir. The absent temp dir then meant the
wiped extensions were never restored, breaking 'flutter build web'
(unresolved web plugins such as audioplayers_web, camera_web, etc.).

Track when the package step ran with --skip-site-packages and skip the
wipe/move in that case, keeping the previous build's extension copy. A
removed extension changes the package requirements, flips the hash, and
takes the full (non-skip) path that still clears stale extensions.

Regression from #6608.

Author: FeodorFitsner

sdk/python/packages/flet-cli/src/flet_cli/commands/build_base.py

@@ -89,6 +89,7 @@ def __init__(self, parser: argparse.ArgumentParser) -> None:
         self.flutter_dir: Optional[Path] = None
         self.flutter_packages_dir = None
         self.flutter_packages_temp_dir = None
+        self.site_packages_skipped = False
         self.platforms = {
             "windows": {
                 "package_platform": "Windows",
@@ -1637,11 +1638,20 @@ def register_flutter_extensions(self):
         # when the app has no Flutter extensions — so always clear the old copy
         # first, otherwise an extension removed since the previous build (e.g.
         # dropping flet-video) would linger here and stay in the built app.
-        if self.flutter_packages_dir.exists():
-            shutil.rmtree(self.flutter_packages_dir, ignore_errors=True)
-        if self.flutter_packages_temp_dir.exists():
-            # copy packages from temp to permanent location
-            shutil.move(self.flutter_packages_temp_dir, self.flutter_packages_dir)
+        #
+        # Skip this when the package step ran with --skip-site-packages: in that
+        # mode serious_python does not repopulate the temp dir, so an absent temp
+        # dir means "unchanged" rather than "no extensions". Wiping here would
+        # delete the previous build's extensions and never restore them, breaking
+        # the Flutter build (unresolved web plugins). A removed extension changes
+        # the package requirements, flips the package hash, and takes the full
+        # (non-skip) path above instead.
+        if not self.site_packages_skipped:
+            if self.flutter_packages_dir.exists():
+                shutil.rmtree(self.flutter_packages_dir, ignore_errors=True)
+            if self.flutter_packages_temp_dir.exists():
+                # copy packages from temp to permanent location
+                shutil.move(self.flutter_packages_temp_dir, self.flutter_packages_dir)
 
         if self.flutter_packages_dir.exists():
             self.update_status("[bold blue]Registering Flutter user extensions...")
@@ -2380,6 +2390,11 @@ def package_python_app(self):
         if not dev_packages_configured:
             if not hash.has_changed():
                 package_args.append("--skip-site-packages")
+                # serious_python skips copying Flutter packages to the temp dir
+                # under --skip-site-packages, so register_flutter_extensions must
+                # keep (not wipe) the permanent flutter-packages copy from the
+                # previous build.
+                self.site_packages_skipped = True
             else:
                 if self.flutter_packages_dir.exists():
                     shutil.rmtree(self.flutter_packages_dir, ignore_errors=True)