Add a migrator to remove FlutterMultiDexApplication.java (#148515)

Fixes https://github.com/flutter/flutter/issues/148368.

See [my comment](https://github.com/flutter/flutter/issues/148368#issuecomment-2116133180) for the specific context.

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

@@ -37,6 +37,7 @@ 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/multidex_removal_migration.dart';
 import 'migrations/top_level_gradle_build_file_migration.dart';
 
 /// The regex to grab variant names from printBuildVariants gradle task
@@ -313,6 +314,7 @@ class AndroidGradleBuilder implements AndroidBuilder {
           androidStudio: _androidStudio,
           java: globals.java),
       MinSdkVersionMigration(project.android, _logger),
+      MultidexRemovalMigration(project.android, _logger),
     ];
 
     final ProjectMigration migration = ProjectMigration(migrators);

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

@@ -0,0 +1,40 @@
+// 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 '../../base/file_system.dart';
+import '../../base/project_migrator.dart';
+import '../../project.dart';
+
+/// This migrator deletes an old file that was created to support enabling
+/// Multidex, which is no longer needed and causes builds to fail if not
+/// deleted.
+class MultidexRemovalMigration extends ProjectMigrator {
+
+  MultidexRemovalMigration(AndroidProject project, super.logger)
+      : _project = project;
+
+  final AndroidProject _project;
+
+  static const String deletionMessage = 'Deleted obsolete FlutterMultiDexApplication.java file.';
+
+  File _getMultiDexApplicationFile() {
+    return _project.hostAppGradleRoot
+        .childDirectory('src')
+        .childDirectory('main')
+        .childDirectory('java')
+        .childDirectory('io')
+        .childDirectory('flutter')
+        .childDirectory('app')
+        .childFile('FlutterMultiDexApplication.java');
+  }
+
+  @override
+  Future<void> migrate() async {
+    final File multiDexApplicationFile = _getMultiDexApplicationFile();
+    if (multiDexApplicationFile.existsSync()) {
+      multiDexApplicationFile.deleteSync();
+      logger.printTrace(deletionMessage);
+    }
+  }
+}

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

@@ -8,6 +8,7 @@ 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/multidex_removal_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';
@@ -426,6 +427,51 @@ tasks.register("clean", Delete) {
         expect(project.appGradleFile.readAsStringSync(), sampleModuleGradleBuildFile(equalsSyntaxMinSdkVersion19));
       });
     });
+
+    group('delete FlutterMultiDexApplication.java, if it exists', ()
+    {
+      late MemoryFileSystem memoryFileSystem;
+      late BufferLogger bufferLogger;
+      late FakeAndroidProject project;
+      late MultidexRemovalMigration 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 = MultidexRemovalMigration(
+            project,
+            bufferLogger
+        );
+      });
+
+      testWithoutContext('do nothing when FlutterMultiDexApplication.java is not present', () async {
+        await migration.migrate();
+        expect(bufferLogger.traceText, isEmpty);
+      });
+
+      testWithoutContext('delete and note when FlutterMultiDexApplication.java is present', () async {
+        // Write a blank string to the FlutterMultiDexApplication.java file.
+        final File flutterMultiDexApplication =  project.hostAppGradleRoot
+            .childDirectory('src')
+            .childDirectory('main')
+            .childDirectory('java')
+            .childDirectory('io')
+            .childDirectory('flutter')
+            .childDirectory('app')
+            .childFile('FlutterMultiDexApplication.java')
+          ..createSync(recursive: true);
+        flutterMultiDexApplication.writeAsStringSync('');
+
+        await migration.migrate();
+        expect(bufferLogger.traceText, contains(MultidexRemovalMigration.deletionMessage));
+        expect(flutterMultiDexApplication.existsSync(), false);
+      });
+    });
   });
 }