Refactor Flutter Gradle Plugin so it can be applied using the declarative plugins {} block (#123511)

This PR aims to resolve #121552.

Resources used:
- [Developing Plugins](https://docs.gradle.org/current/userguide/custom_plugins.html)
- [Using Gradle Plugins](https://docs.gradle.org/current/userguide/plugins.html#sec:plugins_block)
- [Composite Builds Plugin Development Sample](https://docs.gradle.org/current/samples/sample_composite_builds_plugin_development.html)

This PR also paves way for #121541, because apps will no longer have:

```groovy
apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
```

hardcoded. Instead, they'll use:

```groovy
plugins {
    // ...
    id "dev.flutter.flutter-gradle-plugin" // the exact name is tentative
}
```

examples/hello_world/android/app/build.gradle

@@ -2,6 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+plugins {
+    id "com.android.application"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+
 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
@@ -10,11 +15,6 @@ if (localPropertiesFile.exists()) {
     }
 }
 
-def flutterRoot = localProperties.getProperty('flutter.sdk')
-if (flutterRoot == null) {
-    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
-}
-
 def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
 if (flutterVersionCode == null) {
     flutterVersionCode = '1'
@@ -25,9 +25,6 @@ if (flutterVersionName == null) {
     flutterVersionName = '1.0'
 }
 
-apply plugin: 'com.android.application'
-apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
-
 android {
     namespace "io.flutter.examples.hello_world"
     compileSdkVersion flutter.compileSdkVersion

examples/hello_world/android/settings.gradle

@@ -6,16 +6,27 @@
 // To update all the settings.gradle files in the Flutter repo,
 // See dev/tools/bin/generate_gradle_lockfiles.dart.
 
+pluginManagement {
+    def flutterSdkPath = {
+        def properties = new Properties()
+        file("local.properties").withInputStream { properties.load(it) }
+        def flutterSdkPath = properties.getProperty("flutter.sdk")
+        assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
+        return flutterSdkPath
+    }
+    settings.ext.flutterSdkPath = flutterSdkPath()
+
+    // Flutter Gradle Plugin ships together with the Flutter SDK
+    includeBuild("${settings.ext.flutterSdkPath}/packages/flutter_tools/gradle")
+
+    plugins {
+        // Flutter Gradle Plugin's ID is defined in /packages/flutter_tools/gradle/build.gradle.kts.
+        // We set `apply false`, because we don't want to apply the plugin to
+        // the `android` root project, but only to the `app` subproject.
+        id "dev.flutter.flutter-gradle-plugin" version "1.0.0" apply false
+    }
+}
+
 include ':app'
 
-enableFeaturePreview('ONE_LOCKFILE_PER_PROJECT')
-
-def localPropertiesFile = new File(rootProject.projectDir, "local.properties")
-def properties = new Properties()
-
-assert localPropertiesFile.exists()
-localPropertiesFile.withReader("UTF-8") { reader -> properties.load(reader) }
-
-def flutterSdkPath = properties.getProperty("flutter.sdk")
-assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
-apply from: "$flutterSdkPath/packages/flutter_tools/gradle/app_plugin_loader.gradle"
+apply from: "${settings.ext.flutterSdkPath}/packages/flutter_tools/gradle/app_plugin_loader.gradle"

packages/flutter_tools/gradle/build.gradle.kts

@@ -0,0 +1,34 @@
+// 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.
+
+plugins {
+    `groovy-gradle-plugin`
+}
+
+repositories {
+    google()
+    mavenCentral()
+}
+
+
+group = "dev.flutter.plugin"
+version = "1.0.0"
+
+gradlePlugin {
+    plugins {
+        // The "flutterPlugin" name isn't used anywhere.
+        create("flutterPlugin") {
+            id = "dev.flutter.flutter-gradle-plugin"
+            implementationClass = "FlutterPlugin"
+        }
+    }
+}
+
+dependencies {
+    // When bumping, also update:
+    //  * ndkVersion in FlutterExtension in packages/flutter_tools/gradle/src/main/flutter.groovy
+    //  * AGP version constants in packages/flutter_tools/lib/src/android/gradle_utils.dart
+    //  * AGP version in buildscript block in packages/flutter_tools/gradle/src/main/flutter.groovy
+    compileOnly("com.android.tools.build:gradle:7.3.0")
+}

packages/flutter_tools/lib/src/android/gradle_utils.dart

@@ -7,7 +7,6 @@ import 'package:process/process.dart';
 
 import '../base/common.dart';
 import '../base/file_system.dart';
-
 import '../base/logger.dart';
 import '../base/os.dart';
 import '../base/platform.dart';
@@ -33,7 +32,7 @@ const String templateAndroidGradlePluginVersion = '7.3.0';
 const String templateDefaultGradleVersionForModule = '7.3.0';
 const String templateKotlinGradlePluginVersion = '1.7.10';
 
-// These versions should match the values in flutter.gradle (FlutterExtension).
+// These versions should match the values in Flutter Gradle Plugin (FlutterExtension).
 // The Flutter Gradle plugin is only applied to app projects, and modules that are built from source
 // using (include_flutter.groovy).
 // The remaining projects are: plugins, and modules compiled as AARs. In modules, the ephemeral directory

packages/flutter_tools/lib/src/build_info.dart

@@ -217,7 +217,7 @@ class BuildInfo {
   String get modeName => mode.cliName;
   String get friendlyModeName => getFriendlyModeName(mode);
 
-  /// the flavor name in the output apk files is lower-cased (see flutter.gradle),
+  /// the flavor name in the output apk files is lower-cased (see Flutter Gradle Plugin),
   /// so the lower cased flavor name is used to compute the output file name
   String? get lowerCasedFlavor => flavor?.toLowerCase();
 

packages/flutter_tools/lib/src/project.dart

@@ -442,7 +442,8 @@ class AndroidProject extends FlutterProjectPlatform {
 
   static final RegExp _androidNamespacePattern = RegExp('android {[\\S\\s]+namespace[\\s]+[\'"](.+)[\'"]');
   static final RegExp _applicationIdPattern = RegExp('^\\s*applicationId\\s+[\'"](.*)[\'"]\\s*\$');
-  static final RegExp _kotlinPluginPattern = RegExp('^\\s*apply plugin\\:\\s+[\'"]kotlin-android[\'"]\\s*\$');
+  static final RegExp _imperativeKotlinPluginPattern = RegExp('^\\s*apply plugin\\:\\s+[\'"]kotlin-android[\'"]\\s*\$');
+  static final RegExp _declarativeKotlinPluginPattern = RegExp('^\\s*id\\s+[\'"]kotlin-android[\'"]\\s*\$');
   static final RegExp _groupPattern = RegExp('^\\s*group\\s+[\'"](.*)[\'"]\\s*\$');
 
   /// The Gradle root directory of the Android host app. This is the directory
@@ -495,8 +496,10 @@ class AndroidProject extends FlutterProjectPlatform {
       return false;
     }
     for (final String line in appGradle.readAsLinesSync()) {
-      if (line.contains(RegExp(r'apply from: .*/flutter.gradle')) ||
-          line.contains("def flutterPluginVersion = 'managed'")) {
+      final bool fileBasedApply = line.contains(RegExp(r'apply from: .*/flutter.gradle'));
+      final bool declarativeApply = line.contains('dev.flutter.flutter-gradle-plugin');
+      final bool managed = line.contains("def flutterPluginVersion = 'managed'");
+      if (fileBasedApply || declarativeApply || managed) {
         return true;
       }
     }
@@ -506,7 +509,9 @@ class AndroidProject extends FlutterProjectPlatform {
   /// True, if the app project is using Kotlin.
   bool get isKotlin {
     final File gradleFile = hostAppGradleRoot.childDirectory('app').childFile('build.gradle');
-    return firstMatchInFile(gradleFile, _kotlinPluginPattern) != null;
+    final bool imperativeMatch = firstMatchInFile(gradleFile, _imperativeKotlinPluginPattern) != null;
+    final bool declarativeMatch = firstMatchInFile(gradleFile, _declarativeKotlinPluginPattern) != null;
+    return imperativeMatch || declarativeMatch;
   }
 
   File get appManifestFile {

packages/flutter_tools/templates/app_shared/android-java.tmpl/app/build.gradle.tmpl

@@ -1,3 +1,9 @@
+plugins {
+    id "com.android.application"
+    id "kotlin-android"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+
 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
@@ -6,11 +12,6 @@ if (localPropertiesFile.exists()) {
     }
 }
 
-def flutterRoot = localProperties.getProperty('flutter.sdk')
-if (flutterRoot == null) {
-    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
-}
-
 def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
 if (flutterVersionCode == null) {
     flutterVersionCode = '1'
@@ -21,10 +22,6 @@ if (flutterVersionName == null) {
     flutterVersionName = '1.0'
 }
 
-apply plugin: 'com.android.application'
-apply plugin: 'kotlin-android'
-apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
-
 android {
     namespace "{{androidIdentifier}}"
     compileSdkVersion flutter.compileSdkVersion

packages/flutter_tools/templates/app_shared/android-kotlin.tmpl/app/build.gradle.tmpl

@@ -1,3 +1,9 @@
+plugins {
+    id "com.android.application"
+    id "kotlin-android"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+
 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
@@ -6,11 +12,6 @@ if (localPropertiesFile.exists()) {
     }
 }
 
-def flutterRoot = localProperties.getProperty('flutter.sdk')
-if (flutterRoot == null) {
-    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
-}
-
 def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
 if (flutterVersionCode == null) {
     flutterVersionCode = '1'
@@ -21,10 +22,6 @@ if (flutterVersionName == null) {
     flutterVersionName = '1.0'
 }
 
-apply plugin: 'com.android.application'
-apply plugin: 'kotlin-android'
-apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
-
 android {
     namespace "{{androidIdentifier}}"
     compileSdkVersion flutter.compileSdkVersion

packages/flutter_tools/templates/app_shared/android.tmpl/settings.gradle

@@ -1,11 +1,20 @@
-include ':app'
+pluginManagement {
+    def flutterSdkPath = {
+        def properties = new Properties()
+        file("local.properties").withInputStream { properties.load(it) }
+        def flutterSdkPath = properties.getProperty("flutter.sdk")
+        assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
+        return flutterSdkPath
+    }
+    settings.ext.flutterSdkPath = flutterSdkPath()
 
-def localPropertiesFile = new File(rootProject.projectDir, "local.properties")
-def properties = new Properties()
+    includeBuild("${settings.ext.flutterSdkPath}/packages/flutter_tools/gradle")
 
-assert localPropertiesFile.exists()
-localPropertiesFile.withReader("UTF-8") { reader -> properties.load(reader) }
+    plugins {
+        id "dev.flutter.flutter-gradle-plugin" version "1.0.0" apply false
+    }
+}
 
-def flutterSdkPath = properties.getProperty("flutter.sdk")
-assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
-apply from: "$flutterSdkPath/packages/flutter_tools/gradle/app_plugin_loader.gradle"
+include ":app"
+
+apply from: "${settings.ext.flutterSdkPath}/packages/flutter_tools/gradle/app_plugin_loader.gradle"