[reland] Convert the Flutter Gradle Plugin entirely to Kotlin source (flutter#166676)

Relands flutter#166114.

The original PR failed this postsubmit

https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8718287794116896097/+/u/run_engine_dependency_proxy_test/stdout

Because 
```
Task result:
{
  "success": false,
  "reason": "Task failed: Expected Android engine maven dependency URL to resolve to https://storage.googleapis.com/download.flutter.io. Got https://storage.googleapis.com//download.flutter.io instead"
}
```
which was because apparently in Groovy
```groovy
String foo = ""
if (foo) { 
    // branch
}
```
Evaluates to false (i.e. does not take the branch). So we need to check
if the `engineRealm` string is empty, which is what the additional
commit does.

## 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 `///`).
- [x] 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.

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on [Discord].

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---------

Co-authored-by: Gray Mackall <mackall@google.com>

Author: gmackall

packages/flutter_tools/gradle/build.gradle.kts

@@ -34,7 +34,7 @@ gradlePlugin {
         // The "flutterPlugin" name isn't used anywhere.
         create("flutterPlugin") {
             id = "dev.flutter.flutter-gradle-plugin"
-            implementationClass = "FlutterPlugin"
+            implementationClass = "com.flutter.gradle.FlutterPlugin"
         }
         // The "flutterAppPluginLoaderPlugin" name isn't used anywhere.
         create("flutterAppPluginLoaderPlugin") {

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

@@ -25,7 +25,6 @@ object FlutterPluginConstants {
     const val INTERMEDIATES_DIR = "intermediates"
     const val FLUTTER_STORAGE_BASE_URL = "FLUTTER_STORAGE_BASE_URL"
     const val DEFAULT_MAVEN_HOST = "https://storage.googleapis.com"
-    const val WEBSITE_DEPLOYMENT_ANDROID_BUILD_CONFIG = "https://flutter.dev/to/review-gradle-config"
 
     /** Maps platforms to ABI architectures. */
     @JvmStatic val PLATFORM_ARCH_MAP =
@@ -42,7 +41,7 @@ object FlutterPluginConstants {
      * Otherwise, the Play Store will complain that the APK variants have the same version.
      */
     @JvmStatic val ABI_VERSION =
-        mapOf<String, Int>( // Explicit type for clarity, though inferred
+        mapOf<String, Int>(
             ARCH_ARM32 to 1,
             ARCH_ARM64 to 2,
             ARCH_X86 to 3,

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

@@ -10,6 +10,7 @@ import com.android.build.gradle.api.ApplicationVariant
 import com.android.build.gradle.api.BaseVariantOutput
 import com.android.build.gradle.tasks.ProcessAndroidResources
 import com.android.builder.model.BuildType
+import com.flutter.gradle.plugins.PluginHandler
 import groovy.lang.Closure
 import groovy.util.Node
 import groovy.util.XmlParser
@@ -139,19 +140,6 @@ object FlutterPluginUtils {
 
     // TODO(54566): Can remove this function and its call sites once resolved.
 
-    /**
-     * Returns `true` if the given project is a plugin project having an `android` directory
-     * containing a `build.gradle` or `build.gradle.kts` file.
-     */
-    @JvmStatic
-    @JvmName("pluginSupportsAndroidPlatform")
-    internal fun pluginSupportsAndroidPlatform(project: Project): Boolean {
-        val buildGradle = File(File(project.projectDir.parentFile, "android"), "build.gradle")
-        val buildGradleKts =
-            File(File(project.projectDir.parentFile, "android"), "build.gradle.kts")
-        return buildGradle.exists() || buildGradleKts.exists()
-    }
-
     /**
      * Returns the Gradle settings script for the build. When both Groovy and
      * Kotlin variants exist, then Groovy (settings.gradle) is preferred over
@@ -406,7 +394,7 @@ object FlutterPluginUtils {
         return project.property(PROP_LOCAL_ENGINE_BUILD_MODE) == flutterBuildMode
     }
 
-    private fun getAndroidExtension(project: Project): BaseExtension {
+    internal fun getAndroidExtension(project: Project): BaseExtension {
         // Common supertype of the android extension types.
         // But maybe this should be https://developer.android.com/reference/tools/gradle-api/8.7/com/android/build/api/dsl/TestedExtension.
         return project.extensions.findByType(BaseExtension::class.java)!!
@@ -611,7 +599,7 @@ object FlutterPluginUtils {
 
         // Otherwise, point to an empty CMakeLists.txt, and ignore associated warnings.
         gradleProjectAndroidExtension.externalNativeBuild.cmake.path(
-            "$flutterSdkRootPath/packages/flutter_tools/gradle/src/main/groovy/CMakeLists.txt"
+            "$flutterSdkRootPath/packages/flutter_tools/gradle/src/main/scripts/CMakeLists.txt"
         )
 
         // AGP defaults to outputting build artifacts in `android/app/.cxx`. This directory is a
@@ -656,7 +644,7 @@ object FlutterPluginUtils {
     internal fun addFlutterDependencies(
         project: Project,
         buildType: BuildType,
-        pluginList: List<Map<String?, Any?>>,
+        pluginHandler: PluginHandler,
         engineVersion: String
     ) {
         val flutterBuildMode: String = buildModeFor(buildType)
@@ -676,11 +664,9 @@ object FlutterPluginUtils {
         // embedding.
         val pluginsThatIncludeFlutterEmbeddingAsTransitiveDependency: List<Map<String?, Any?>> =
             if (flutterBuildMode == "release") {
-                getPluginListWithoutDevDependencies(
-                    pluginList
-                )
+                pluginHandler.getPluginListWithoutDevDependencies()
             } else {
-                pluginList
+                pluginHandler.getPluginList()
             }
 
         if (!isFlutterAppProject(project) || pluginsThatIncludeFlutterEmbeddingAsTransitiveDependency.isEmpty()) {
@@ -702,143 +688,6 @@ object FlutterPluginUtils {
         }
     }
 
-    /**
-     * Gets the list of plugins (as map) that support the Android platform and are dependencies of the
-     * Android project excluding dev dependencies.
-     *
-     * 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/scripts/native_plugin_loader.gradle.kts
-     */
-    private fun getPluginListWithoutDevDependencies(pluginList: List<Map<String?, Any?>>): List<Map<String?, Any?>> =
-        pluginList.filter { pluginObject -> pluginObject["dev_dependency"] == false }
-
-    /**
-     * Add the dependencies on other plugin projects to the plugin project.
-     * A plugin A can depend on plugin B. As a result, this dependency must be surfaced by
-     * making the Gradle plugin project A depend on the Gradle plugin project B.
-     */
-    @JvmStatic
-    @JvmName("configurePluginDependencies")
-    internal fun configurePluginDependencies(
-        project: Project,
-        pluginObject: Map<String?, Any?>
-    ) {
-        val pluginName: String =
-            requireNotNull(pluginObject["name"] as? String) {
-                "Missing valid \"name\" property for plugin object: $pluginObject"
-            }
-        val pluginProject: Project = project.rootProject.findProject(":$pluginName") ?: return
-
-        getAndroidExtension(project).buildTypes.forEach { buildType ->
-            val flutterBuildMode: String = buildModeFor(buildType)
-            if (flutterBuildMode == "release" && (pluginObject["dev_dependency"] as? Boolean == true)) {
-                // This plugin is a dev dependency will not be included in the
-                // release build, so no need to add its dependencies.
-                return@forEach
-            }
-            val dependencies = requireNotNull(pluginObject["dependencies"] as? List<*>)
-            dependencies.forEach innerForEach@{ pluginDependencyName ->
-                check(pluginDependencyName is String)
-                if (pluginDependencyName.isEmpty()) {
-                    return@innerForEach
-                }
-
-                val dependencyProject =
-                    project.rootProject.findProject(":$pluginDependencyName") ?: return@innerForEach
-                pluginProject.afterEvaluate {
-                    pluginProject.dependencies.add("implementation", dependencyProject)
-                }
-            }
-        }
-    }
-
-    /**
-     * Performs configuration related to the plugin's Gradle [Project], including
-     * 1. Adding the plugin itself as a dependency to the main project.
-     * 2. Adding the main project's build types to the plugin's build types.
-     * 3. Adding a dependency on the Flutter embedding to the plugin.
-     *
-     * Should only be called on plugins that support the Android platform.
-     */
-    @JvmStatic
-    @JvmName("configurePluginProject")
-    internal fun configurePluginProject(
-        project: Project,
-        pluginObject: Map<String?, Any?>,
-        engineVersion: String
-    ) {
-        // TODO(gmackall): should guard this with a pluginObject.contains().
-        val pluginName =
-            requireNotNull(pluginObject["name"] as? String) { "Plugin name must be a string for plugin object: $pluginObject" }
-        val pluginProject: Project = project.rootProject.findProject(":$pluginName") ?: return
-
-        // Apply the "flutter" Gradle extension to plugins so that they can use it's vended
-        // compile/target/min sdk values.
-        pluginProject.extensions.create("flutter", FlutterExtension::class.java)
-
-        // Add plugin dependency to the app project. We only want to add dependency
-        // for dev dependencies in non-release builds.
-        project.afterEvaluate {
-            getAndroidExtension(project).buildTypes.forEach { buildType ->
-                if (!(pluginObject["dev_dependency"] as Boolean) || buildType.name != "release") {
-                    project.dependencies.add("${buildType.name}Api", pluginProject)
-                }
-            }
-        }
-
-        // Wait until the Android plugin loaded.
-        pluginProject.afterEvaluate {
-            // Checks if there is a mismatch between the plugin compileSdkVersion and the project compileSdkVersion.
-            val projectCompileSdkVersion: String = getCompileSdkFromProject(project)
-            val pluginCompileSdkVersion: String = getCompileSdkFromProject(pluginProject)
-            // TODO(gmackall): This is doing a string comparison, which is odd and also can be wrong
-            //                 when comparing preview versions (against non preview, and also in the
-            //                 case of alphabet reset which happened with "Baklava".
-            if (pluginCompileSdkVersion > projectCompileSdkVersion) {
-                project.logger.quiet("Warning: The plugin $pluginName requires Android SDK version $pluginCompileSdkVersion or higher.")
-                project.logger.quiet(
-                    "For more information about build configuration, see ${FlutterPluginConstants.WEBSITE_DEPLOYMENT_ANDROID_BUILD_CONFIG}."
-                )
-            }
-
-            getAndroidExtension(project).buildTypes.forEach { buildType ->
-                addEmbeddingDependencyToPlugin(project, pluginProject, buildType, engineVersion)
-            }
-        }
-    }
-
-    private fun addEmbeddingDependencyToPlugin(
-        project: Project,
-        pluginProject: Project,
-        buildType: BuildType,
-        engineVersion: String
-    ) {
-        val flutterBuildMode: String = buildModeFor(buildType)
-        // TODO(gmackall): this should be safe to remove, as the minimum required AGP is well above
-        //                 3.5. We should try to remove it.
-        // In AGP 3.5, the embedding must be added as an API implementation,
-        // so java8 features are desugared against the runtime classpath.
-        // For more, see https://github.com/flutter/flutter/issues/40126
-        if (!supportsBuildMode(pluginProject, flutterBuildMode)) {
-            return
-        }
-        if (!pluginProject.hasProperty("android")) {
-            return
-        }
-
-        // Copy build types from the app to the plugin.
-        // This allows to build apps with plugins and custom build types or flavors.
-        getAndroidExtension(pluginProject).buildTypes.addAll(getAndroidExtension(project).buildTypes)
-
-        // The embedding is API dependency of the plugin, so the AGP is able to desugar
-        // default method implementations when the interface is implemented by a plugin.
-        //
-        // See https://issuetracker.google.com/139821726, and
-        // https://github.com/flutter/flutter/issues/72185 for more details.
-        addApiDependencies(pluginProject, buildType.name, "io.flutter:flutter_embedding_$flutterBuildMode:$engineVersion")
-    }
-
     // ------------------ Task adders (a subset of the above category)
 
     // Add a task that can be called on flutter projects that prints the Java version used in Gradle.

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

@@ -17,41 +17,37 @@ import java.io.File
  */
 
 object NativePluginLoaderReflectionBridge {
-    private var nativePluginLoader: Any? = null
-
     /**
      * An abstraction to hide reflection from calling sites. See ../scripts/native_plugin_loader.gradle.kts.
      */
-    @JvmStatic
     fun getPlugins(
         extraProperties: ExtraPropertiesExtension,
         flutterProjectRoot: File
-    ): List<Map<String, Any>> {
-        nativePluginLoader = extraProperties.get("nativePluginLoader")!!
+    ): List<Map<String?, Any?>> {
+        val nativePluginLoader = extraProperties.get("nativePluginLoader")!!
 
         @Suppress("UNCHECKED_CAST")
-        val pluginList: List<Map<String, Any>> =
-            nativePluginLoader!!::class
+        val pluginList: List<Map<String?, Any?>> =
+            nativePluginLoader::class
                 .members
                 .firstOrNull { it.name == "getPlugins" }
-                ?.call(nativePluginLoader, flutterProjectRoot) as List<Map<String, Any>>
+                ?.call(nativePluginLoader, flutterProjectRoot) as List<Map<String?, Any?>>
 
         return pluginList
     }
 
     /**
      * An abstraction to hide reflection from calling sites. See ../scripts/native_plugin_loader.gradle.kts.
      */
-    @JvmStatic
     fun getDependenciesMetadata(
         extraProperties: ExtraPropertiesExtension,
         flutterProjectRoot: File
     ): Map<String, Any> {
-        nativePluginLoader = extraProperties.get("nativePluginLoader")!!
+        val nativePluginLoader = extraProperties.get("nativePluginLoader")!!
 
         @Suppress("UNCHECKED_CAST")
         val dependenciesMetadata: Map<String, Any> =
-            nativePluginLoader!!::class
+            nativePluginLoader::class
                 .members
                 .firstOrNull { it.name == "dependenciesMetadata" }
                 ?.call(nativePluginLoader, flutterProjectRoot) as Map<String, Any>