platform upgrades

android/build.gradle

@@ -5,10 +5,25 @@ allprojects {
     }
 }
 
-rootProject.buildDir = '../build'
+rootProject.layout.buildDirectory = file('../build')
 subprojects {
-    project.buildDir = "${rootProject.buildDir}/${project.name}"
+    project.layout.buildDirectory = rootProject.layout.buildDirectory.dir(project.name)
 }
+// AGP 9 enables built-in Kotlin, but Flutter's migrator pins
+// android.builtInKotlin=false while the plugin ecosystem catches up. Most
+// plugins honour that flag and fall back to KGP themselves; file_picker and
+// background_downloader only check the AGP major version and silently skip
+// applying KGP, which leaves their Kotlin sources uncompiled (missing
+// FilePickerPlugin/BackgroundDownloaderPlugin at link time). Apply KGP for
+// them as soon as AGP is on the project, i.e. before their android {} block
+// runs. Drop this once both plugins ship a builtInKotlin-aware build script.
+gradle.beforeProject { sub ->
+    if (!(sub.name in ['file_picker', 'background_downloader'])) return
+    sub.plugins.withId('com.android.library') {
+        sub.pluginManager.apply('org.jetbrains.kotlin.android')
+    }
+}
+
 // Pin every Android subproject to JVM 17 so plugins that ship Kotlin sources
 // compiled with a higher target (e.g. receive_sharing_intent at 21) or stale
 // Java compatibility (e.g. home_widget at 1.8) don't break the build under
@@ -42,5 +57,5 @@ subprojects {
 }
 
 tasks.register("clean", Delete) {
-    delete rootProject.buildDir
+    delete rootProject.layout.buildDirectory
 }