Convert AppPluginLoaderPlugin to Kotlin, and add NativePluginLoaderReflectionBridge to expose it in Kotlin (flutter#166027)

Graph [stolen from Barteks
comment](flutter#161352 (comment))
documenting the existing (pre pr) state:

```mermaid
graph TD;
  flutter.groovy -- import --> native_plugin_loader.groovy;
  flutter.groovy -- import --> BaseApplicationNameHandler.kt;
  module_plugin_loader.groovy -- "ext" --> native_plugin_loader.groovy;
  app_plugin_loader.groovy  -- import --> native_plugin_loader.groovy;
  include_flutter.groovy -- "apply from: " --> module_plugin_loader.groovy;
```
1. Converts the `app_plugin_loader.groovy` to kotlin source. 
2. Converts the `module_plugin_loader.groovy` to kotlin script. This
can't be changed to kotlin source yet, as we will need to instruct users
to make a change to their host app-level gradle files before we can turn
down script application of this separate gradle plugin. This is a
breaking change, and will need a quarter at least of notice.
3. Unfortunately, the main Flutter Gradle plugin depends on being able
to call methods of the `native_plugin_loader`, which we could do in
groovy via wacky dynamic behavior, calling across the compiled
plugin->script plugin barrier. We can't do this in Kotlin source, and we
also can't fully convert `native_plugin_loader` to kotlin source yet
because of (2), so I've added a `NativePluginLoaderReflectionBridge`
that allows us to access the methods in the
`native_plugin_loader.gradle.kts` from the Kotlin source files, calling
across the compiled plugin->script plugin barrier as we were before.

The plan here is
1. to follow up by adding a converted `native_plugin_loader.gradle.kts`
in Kotlin source, and migrating all paths but the host-app using a
module-as-source to use the converted approach (but not deleting the old
way)
2. maintaining both ways for a release or two, with the script
application printing a message notifying users to update to the
non-script based application of the `module_plugin_loader`.
3. Then we can delete the script based apply, and also the reflection
bridge.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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[Tree Hygiene]:
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[Discord]:
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[Data Driven Fixes]:
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---------

Co-authored-by: Gray Mackall <[email protected]>

Author: gmackall

packages/flutter_tools/gradle/build.gradle.kts

@@ -39,7 +39,7 @@ gradlePlugin {
         // The "flutterAppPluginLoaderPlugin" name isn't used anywhere.
         create("flutterAppPluginLoaderPlugin") {
             id = "dev.flutter.flutter-plugin-loader"
-            implementationClass = "FlutterAppPluginLoaderPlugin"
+            implementationClass = "com.flutter.gradle.FlutterAppPluginLoaderPlugin"
         }
     }
 }

packages/flutter_tools/gradle/module_plugin_loader.gradle

@@ -8,7 +8,7 @@
 import java.nio.file.Paths
 
 File pathToThisDirectory = buildscript.sourceFile.parentFile
-apply from: Paths.get(pathToThisDirectory.absolutePath, "src", "main", "groovy", "native_plugin_loader.groovy")
+apply from: Paths.get(pathToThisDirectory.absolutePath, "src", "main", "scripts", "native_plugin_loader.gradle.kts")
 
 def moduleProjectRoot = project(':flutter').projectDir.parentFile.parentFile
 

packages/flutter_tools/gradle/src/main/groovy/app_plugin_loader.groovy

@@ -15,6 +15,7 @@ import com.flutter.gradle.FlutterExtension
 import com.flutter.gradle.FlutterPluginConstants
 import com.flutter.gradle.FlutterTask
 import com.flutter.gradle.FlutterPluginUtils
+import com.flutter.gradle.NativePluginLoaderReflectionBridge
 import org.gradle.api.file.Directory
 
 import java.nio.file.Paths
@@ -114,7 +115,7 @@ class FlutterPlugin implements Plugin<Project> {
         }
 
         // Load shared gradle functions
-        project.apply from: Paths.get(flutterRoot.absolutePath, "packages", "flutter_tools", "gradle", "src", "main", "groovy", "native_plugin_loader.groovy")
+        project.apply from: Paths.get(flutterRoot.absolutePath, "packages", "flutter_tools", "gradle", "src", "main", "scripts", "native_plugin_loader.gradle.kts")
 
         FlutterExtension extension = project.extensions.create("flutter", FlutterExtension)
         Properties localProperties = new Properties()
@@ -298,7 +299,7 @@ class FlutterPlugin implements Plugin<Project> {
      * and filtered then with the [doesSupportAndroidPlatform] method instead of
      * just using the `plugins.android` list.
      */
-    private void configureLegacyPluginEachProjects(Project project) {
+    static private void configureLegacyPluginEachProjects(Project project) {
         try {
             // Read the contents of the settings.gradle file.
             // Remove block/line comments
@@ -340,11 +341,11 @@ class FlutterPlugin implements Plugin<Project> {
      *
      * The map value contains either the plugins `name` (String),
      * its `path` (String), or its `dependencies` (List<String>).
-     * See [NativePluginLoader#getPlugins] in packages/flutter_tools/gradle/src/main/groovy/native_plugin_loader.groovy
+     * See [NativePluginLoader#getPlugins] in packages/flutter_tools/gradle/src/main/scripts/native_plugin_loader.gradle.kts
      */
     private List<Map<String, Object>> getPluginList(Project project) {
         if (pluginList == null) {
-            pluginList = project.ext.nativePluginLoader.getPlugins(FlutterPluginUtils.getFlutterSourceDirectory(project))
+            pluginList = NativePluginLoaderReflectionBridge.getPlugins(project.ext, FlutterPluginUtils.getFlutterSourceDirectory(project))
         }
         return pluginList
     }
@@ -354,7 +355,7 @@ class FlutterPlugin implements Plugin<Project> {
     /** Gets the plugins dependencies from `.flutter-plugins-dependencies`. */
     private List<Map<String, Object>> getPluginDependencies(Project project) {
         if (pluginDependencies == null) {
-            Map meta = project.ext.nativePluginLoader.getDependenciesMetadata(FlutterPluginUtils.getFlutterSourceDirectory(project))
+            Map meta = NativePluginLoaderReflectionBridge.getDependenciesMetadata(project.ext, FlutterPluginUtils.getFlutterSourceDirectory(project))
             if (meta == null) {
                 pluginDependencies = []
             } else {

packages/flutter_tools/gradle/src/main/groovy/flutter.groovy

@@ -1,3 +1,7 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 package com.flutter.gradle
 
 import com.android.build.api.dsl.ApplicationExtension

packages/flutter_tools/gradle/src/main/groovy/native_plugin_loader.groovy

@@ -1,3 +1,7 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 package com.flutter.gradle
 
 import androidx.annotation.VisibleForTesting

packages/flutter_tools/gradle/src/main/kotlin/BaseApplicationNameHandler.kt

@@ -1,3 +1,7 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 package com.flutter.gradle
 
 import androidx.annotation.VisibleForTesting

packages/flutter_tools/gradle/src/main/kotlin/BaseFlutterTaskHelper.kt

@@ -0,0 +1,66 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package com.flutter.gradle
+
+import org.gradle.api.Plugin
+import org.gradle.api.initialization.Settings
+import org.jetbrains.kotlin.gradle.plugin.extraProperties
+import java.io.File
+import java.nio.file.Paths
+import java.util.Properties
+
+private const val FLUTTER_SDK_PATH = "flutterSdkPath"
+
+// Integration tests that cover this class include
+// - packages/flutter_tools/test/integration.shard/android_gradle_daemon_cache_test.dart
+// - packages/flutter_tools/test/integration.shard/android_plugin_compilesdkversion_mismatch_test.dart
+// And can be run by following the README in  packages/flutter_tools/.
+
+/**
+ * This plugin applies the native plugin loader plugin (../scripts/native_plugin_loader.gradle.kts)
+ * and then configures the main project to `include` each of the loaded flutter plugins.
+ */
+class FlutterAppPluginLoaderPlugin : Plugin<Settings> {
+    override fun apply(settings: Settings) {
+        val flutterProjectRoot: File = settings.settingsDir.parentFile
+
+        if (!settings.extraProperties.has(FLUTTER_SDK_PATH)) {
+            val properties = Properties()
+            val localPropertiesFile = File(settings.rootProject.projectDir, "local.properties")
+            localPropertiesFile.inputStream().use { properties.load(it) }
+            settings.extraProperties.set(FLUTTER_SDK_PATH, properties.getProperty("flutter.sdk"))
+            assert(
+                settings.extraProperties.get(FLUTTER_SDK_PATH) != null
+            ) { "flutter.sdk not set in local.properties" }
+        }
+
+        settings.apply {
+            from(
+                Paths.get(
+                    settings.extraProperties.get(FLUTTER_SDK_PATH) as String,
+                    "packages",
+                    "flutter_tools",
+                    "gradle",
+                    "src",
+                    "main",
+                    "scripts",
+                    "native_plugin_loader.gradle.kts"
+                )
+            )
+        }
+
+        NativePluginLoaderReflectionBridge
+            .getPlugins(settings.extraProperties, flutterProjectRoot)
+            .forEach { androidPlugin ->
+                val pluginDirectory = File(androidPlugin["path"] as String, "android")
+                check(
+                    pluginDirectory.exists()
+                ) { "Plugin directory does not exist: ${pluginDirectory.absolutePath}" }
+                val pluginName = androidPlugin["name"] as String
+                settings.include(":$pluginName")
+                settings.project(":$pluginName").projectDir = pluginDirectory
+            }
+    }
+}

packages/flutter_tools/gradle/src/main/kotlin/DependencyVersionChecker.kt

@@ -1,3 +1,7 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 package com.flutter.gradle
 
 import org.gradle.api.GradleException