Add an android migrator to upgrade minSdkVersions 16,17,18 to flutter.minSdkVersion (#129729)

This migrator will upgrade the minSdkVersion used in the [module-level build.gradle](https://developer.android.com/build#module-level) file to flutter.minSdkVersion. 

The PR also makes a small refactor to `AndroidProject` to add a getter for the module level build.gradle file, and uses that getter in places where we were getting that file (previously it was being gotten directly via `hostAppGradleRoot.childDirectory('app').childFile('build.gradle')`.

Part of the work for deprecating support for the Jelly Bean android apis.

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

@@ -35,6 +35,7 @@ import 'gradle_errors.dart';
 import 'gradle_utils.dart';
 import 'java.dart';
 import 'migrations/android_studio_java_gradle_conflict_migration.dart';
+import 'migrations/min_sdk_version_migration.dart';
 import 'migrations/top_level_gradle_build_file_migration.dart';
 import 'multidex.dart';
 
@@ -330,8 +331,8 @@ class AndroidGradleBuilder implements AndroidBuilder {
       AndroidStudioJavaGradleConflictMigration(_logger,
           project: project.android,
           androidStudio: _androidStudio,
-          java: globals.java)
-      ,
+          java: globals.java),
+      MinSdkVersionMigration(project.android, _logger),
     ];
 
     final ProjectMigration migration = ProjectMigration(migrators);

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

@@ -57,23 +57,28 @@ const String maxKnownAgpVersion = '8.1';
 // Parentheticals are use to group which helps with version extraction.
 // "...build:gradle:(...)" where group(1) should be the version string.
 final RegExp _androidGradlePluginRegExp =
-    RegExp(r'com\.android\.tools\.build:gradle:(\d+\.\d+\.\d+)');
+  RegExp(r'com\.android\.tools\.build:gradle:(\d+\.\d+\.\d+)');
 
 // Expected content format (with lines above and below).
 // Version can have 2 or 3 numbers.
 // 'distributionUrl=https\://services.gradle.org/distributions/gradle-7.4.2-all.zip'
 // '^\s*' protects against commented out lines.
 final RegExp distributionUrlRegex =
-    RegExp(r'^\s*distributionUrl\s*=\s*.*\.zip', multiLine: true);
+  RegExp(r'^\s*distributionUrl\s*=\s*.*\.zip', multiLine: true);
 
 // Modified version of the gradle distribution url match designed to only match
 // gradle.org urls so that we can guarantee any modifications to the url
 // still points to a hosted zip.
 final RegExp gradleOrgVersionMatch =
-RegExp(
+  RegExp(
     r'^\s*distributionUrl\s*=\s*https\\://services\.gradle\.org/distributions/gradle-((?:\d|\.)+)-(.*)\.zip',
     multiLine: true
-);
+  );
+
+// This matches uncommented minSdkVersion lines in the module-level build.gradle
+// file which have minSdkVersion 16,17, or 18 (the Jelly Bean api levels).
+final RegExp jellyBeanMinSdkVersionMatch =
+  RegExp(r'(?<=^\s*)minSdkVersion 1[678](?=\s*(?://|$))', multiLine: true);
 
 // From https://docs.gradle.org/current/userguide/command_line_interface.html#command_line_interface
 const String gradleVersionFlag = r'--version';

packages/flutter_tools/lib/src/android/migrations/min_sdk_version_migration.dart

+// 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.
+
+import 'package:meta/meta.dart';
+
+import '../../base/file_system.dart';
+import '../../base/project_migrator.dart';
+import '../../project.dart';
+import '../gradle_utils.dart';
+
+/// Replacement value for https://developer.android.com/reference/tools/gradle-api/8.0/com/android/build/api/dsl/BaseFlavor#minSdkVersion(kotlin.Int)
+/// that instead of using a value defaults to the version defined by the
+/// flutter sdk as the minimum supported by flutter.
+@visibleForTesting
+const String replacementMinSdkText = 'minSdkVersion flutter.minSdkVersion';
+
+@visibleForTesting
+const String appGradleNotFoundWarning = 'Module level build.gradle file not found, skipping minSdkVersion migration.';
+
+class MinSdkVersionMigration extends ProjectMigrator {
+  MinSdkVersionMigration(
+      AndroidProject project,
+      super.logger,
+  ) : _project = project;
+
+  final AndroidProject _project;
+
+  @override
+  void migrate() {
+    // Skip applying migration in modules as the FlutterExtension is not applied.
+    if (_project.isModule) {
+      return;
+    }
+    try {
+      processFileLines(_project.appGradleFile);
+    } on FileSystemException {
+      // Skip if we cannot find the app level build.gradle file.
+      logger.printTrace(appGradleNotFoundWarning);
+    }
+  }
+
+  @override
+  String migrateFileContents(String fileContents) {
+    return fileContents.replaceAll(jellyBeanMinSdkVersionMatch, replacementMinSdkText);
+  }
+}

packages/flutter_tools/lib/src/project.dart

@@ -509,30 +509,45 @@ class AndroidProject extends FlutterProjectPlatform {
     if (plugin.existsSync()) {
       return false;
     }
-    final File appGradle = hostAppGradleRoot.childFile(
-        fileSystem.path.join('app', 'build.gradle'));
-    if (!appGradle.existsSync()) {
-      return false;
-    }
-    for (final String line in appGradle.readAsLinesSync()) {
-      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;
+    try {
+      for (final String line in appGradleFile.readAsLinesSync()) {
+        // This syntax corresponds to applying the Flutter Gradle Plugin with a
+        // script.
+        // See https://docs.gradle.org/current/userguide/plugins.html#sec:script_plugins.
+        final bool fileBasedApply = line.contains(RegExp(r'apply from: .*/flutter.gradle'));
+
+        // This syntax corresponds to applying the Flutter Gradle Plugin using
+        // the declarative "plugins {}" block after including it in the
+        // pluginManagement block of the settings.gradle file.
+        // See https://docs.gradle.org/current/userguide/composite_builds.html#included_plugin_builds,
+        // as well as the settings.gradle and build.gradle templates.
+        final bool declarativeApply = line.contains('dev.flutter.flutter-gradle-plugin');
+
+        // This case allows for flutter run/build to work for modules. It does
+        // not guarantee the Flutter Gradle Plugin is applied.
+        final bool managed = line.contains("def flutterPluginVersion = 'managed'");
+        if (fileBasedApply || declarativeApply || managed) {
+          return true;
+        }
       }
+    } on FileSystemException {
+      return false;
     }
     return false;
   }
 
   /// True, if the app project is using Kotlin.
   bool get isKotlin {
-    final File gradleFile = hostAppGradleRoot.childDirectory('app').childFile('build.gradle');
-    final bool imperativeMatch = firstMatchInFile(gradleFile, _imperativeKotlinPluginPattern) != null;
-    final bool declarativeMatch = firstMatchInFile(gradleFile, _declarativeKotlinPluginPattern) != null;
+    final bool imperativeMatch = firstMatchInFile(appGradleFile, _imperativeKotlinPluginPattern) != null;
+    final bool declarativeMatch = firstMatchInFile(appGradleFile, _declarativeKotlinPluginPattern) != null;
     return imperativeMatch || declarativeMatch;
   }
 
+  /// Gets the module-level build.gradle file.
+  /// See https://developer.android.com/build#module-level.
+  File get appGradleFile => hostAppGradleRoot.childDirectory('app')
+      .childFile('build.gradle');
+
   File get appManifestFile {
     if (isUsingGradle) {
       return hostAppGradleRoot
@@ -627,21 +642,18 @@ $javaGradleCompatUrl
   }
 
   String? get applicationId {
-    final File gradleFile = hostAppGradleRoot.childDirectory('app').childFile('build.gradle');
-    return firstMatchInFile(gradleFile, _applicationIdPattern)?.group(1);
+    return firstMatchInFile(appGradleFile, _applicationIdPattern)?.group(1);
   }
 
   /// Get the namespace for newer Android projects,
   /// which replaces the `package` attribute in the Manifest.xml.
   String? get namespace {
-    final File gradleFile = hostAppGradleRoot.childDirectory('app').childFile('build.gradle');
-
-    if (!gradleFile.existsSync()) {
+    try {
+      // firstMatchInFile() reads per line but `_androidNamespacePattern` matches a multiline pattern.
+      return _androidNamespacePattern.firstMatch(appGradleFile.readAsStringSync())?.group(1);
+    } on FileSystemException {
       return null;
     }
-
-    // firstMatchInFile() reads per line but `_androidNamespacePattern` matches a multiline pattern.
-    return _androidNamespacePattern.firstMatch(gradleFile.readAsStringSync())?.group(1);
   }
 
   String? get group {

packages/flutter_tools/test/general.shard/android/android_project_migration_test.dart

@@ -7,6 +7,7 @@ import 'package:file/memory.dart';
 import 'package:flutter_tools/src/android/android_studio.dart';
 import 'package:flutter_tools/src/android/gradle_utils.dart';
 import 'package:flutter_tools/src/android/migrations/android_studio_java_gradle_conflict_migration.dart';
+import 'package:flutter_tools/src/android/migrations/min_sdk_version_migration.dart';
 import 'package:flutter_tools/src/android/migrations/top_level_gradle_build_file_migration.dart';
 import 'package:flutter_tools/src/base/logger.dart';
 import 'package:flutter_tools/src/base/version.dart';
@@ -41,6 +42,79 @@ zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
 ''';
 
+String sampleModuleGradleBuildFile(String minSdkVersionString) {
+  return r'''
+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()) {
+    localPropertiesFile.withReader('UTF-8') { reader ->
+        localProperties.load(reader)
+    }
+}
+
+def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
+if (flutterVersionCode == null) {
+    flutterVersionCode = '1'
+}
+
+def flutterVersionName = localProperties.getProperty('flutter.versionName')
+if (flutterVersionName == null) {
+    flutterVersionName = '1.0'
+}
+
+android {
+    namespace "com.example.asset_sample"
+    compileSdkVersion flutter.compileSdkVersion
+    ndkVersion flutter.ndkVersion
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
+    kotlinOptions {
+        jvmTarget = '1.8'
+    }
+
+    sourceSets {
+        main.java.srcDirs += 'src/main/kotlin'
+    }
+
+    defaultConfig {
+        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
+        applicationId "com.example.asset_sample"
+        // You can update the following values to match your application needs.
+        // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-gradle-build-configuration.
+        ''' + minSdkVersionString + r'''
+
+        targetSdkVersion flutter.targetSdkVersion
+        versionCode flutterVersionCode.toInteger()
+        versionName flutterVersionName
+    }
+
+    buildTypes {
+        release {
+            // TODO: Add your own signing config for the release build.
+            // Signing with the debug keys for now, so `flutter run --release` works.
+            signingConfig signingConfigs.debug
+        }
+    }
+}
+
+flutter {
+    source '../..'
+}
+
+dependencies {}
+''';
+}
+
 final Version androidStudioDolphin = Version(2021, 3, 1);
 
 const Version _javaVersion17 = Version.withText(17, 0, 2, 'openjdk 17.0.2');
@@ -257,14 +331,128 @@ tasks.register("clean", Delete) {
         expect(bufferLogger.traceText, contains(optOutFlagEnabled));
       });
     });
+
+    group('migrate min sdk versions less than 19 to flutter.minSdkVersion '
+        'when in a FlutterProject that is an app', ()
+    {
+      late MemoryFileSystem memoryFileSystem;
+      late BufferLogger bufferLogger;
+      late FakeAndroidProject project;
+      late MinSdkVersionMigration migration;
+
+      setUp(() {
+        memoryFileSystem = MemoryFileSystem.test();
+        memoryFileSystem.currentDirectory.childDirectory('android').createSync();
+        bufferLogger = BufferLogger.test();
+        project = FakeAndroidProject(
+          root: memoryFileSystem.currentDirectory.childDirectory('android'),
+        );
+        project.appGradleFile.parent.createSync(recursive: true);
+        migration = MinSdkVersionMigration(
+            project,
+            bufferLogger
+        );
+      });
+
+      testWithoutContext('do nothing when files missing', () {
+        migration.migrate();
+        expect(bufferLogger.traceText, contains(appGradleNotFoundWarning));
+      });
+
+      testWithoutContext('replace when api 16', () {
+        const String minSdkVersion16 = 'minSdkVersion 16';
+        project.appGradleFile.writeAsStringSync(sampleModuleGradleBuildFile(minSdkVersion16));
+        migration.migrate();
+        expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(replacementMinSdkText));
+      });
+
+      testWithoutContext('replace when api 17', () {
+        const String minSdkVersion17 = 'minSdkVersion 17';
+        project.appGradleFile.writeAsStringSync(sampleModuleGradleBuildFile(minSdkVersion17));
+        migration.migrate();
+        expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(replacementMinSdkText));
+      });
+
+      testWithoutContext('replace when api 18', () {
+        const String minSdkVersion18 = 'minSdkVersion 18';
+        project.appGradleFile.writeAsStringSync(sampleModuleGradleBuildFile(minSdkVersion18));
+        migration.migrate();
+        expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(replacementMinSdkText));
+      });
+
+      testWithoutContext('do nothing when >=api 19', () {
+        const String minSdkVersion19 = 'minSdkVersion 19';
+        project.appGradleFile.writeAsStringSync(sampleModuleGradleBuildFile(minSdkVersion19));
+        migration.migrate();
+        expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(minSdkVersion19));
+      });
+
+      testWithoutContext('do nothing when already using '
+          'flutter.minSdkVersion', () {
+        project.appGradleFile.writeAsStringSync(sampleModuleGradleBuildFile(replacementMinSdkText));
+        migration.migrate();
+        expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(replacementMinSdkText));
+      });
+
+      testWithoutContext('avoid rewriting comments', () {
+        const String code = '// minSdkVersion 16  // old default\n'
+            '        minSdkVersion 23  // new version';
+        project.appGradleFile.writeAsStringSync(sampleModuleGradleBuildFile(code));
+        migration.migrate();
+        expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(code));
+      });
+
+      testWithoutContext('do nothing when project is a module', () {
+        project = FakeAndroidProject(
+          root: memoryFileSystem.currentDirectory.childDirectory('android'),
+          module: true,
+        );
+        migration = MinSdkVersionMigration(
+            project,
+            bufferLogger
+        );
+        const String minSdkVersion16 = 'minSdkVersion 16';
+        project.appGradleFile.writeAsStringSync(sampleModuleGradleBuildFile(minSdkVersion16));
+        migration.migrate();
+        expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(minSdkVersion16));
+      });
+
+      testWithoutContext('do nothing when minSdkVersion is set '
+          'to a constant', () {
+        const String minSdkVersionConstant = 'minSdkVersion kMinSdkversion';
+        project.appGradleFile.writeAsStringSync(sampleModuleGradleBuildFile(minSdkVersionConstant));
+        migration.migrate();
+        expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(minSdkVersionConstant));
+      });
+
+      testWithoutContext('do nothing when minSdkVersion is set '
+          'using = syntax', () {
+        const String equalsSyntaxMinSdkVersion16 = 'minSdkVersion = 16';
+        project.appGradleFile.writeAsStringSync(sampleModuleGradleBuildFile(equalsSyntaxMinSdkVersion16));
+        migration.migrate();
+        expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(equalsSyntaxMinSdkVersion16));
+      });
+    });
   });
 }
 
 class FakeAndroidProject extends Fake implements AndroidProject {
-  FakeAndroidProject({required Directory root}) : hostAppGradleRoot = root;
+  FakeAndroidProject({required Directory root, this.module, this.plugin}) : hostAppGradleRoot = root;
 
   @override
   Directory hostAppGradleRoot;
+
+  final bool? module;
+  final bool? plugin;
+
+  @override
+  bool get isPlugin => plugin ?? false;
+
+  @override
+  bool get isModule => module ?? false;
+
+  @override
+  File get appGradleFile => hostAppGradleRoot.childDirectory('app').childFile('build.gradle');
 }
 
 class FakeAndroidStudio extends Fake implements AndroidStudio {