Retire v1 embedding compatibility from automatic multidex support (#100685)

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

@@ -25,25 +25,35 @@ File _getMultiDexApplicationFile(Directory projectDir) {
 void ensureMultiDexApplicationExists(final Directory projectDir) {
   final File applicationFile = _getMultiDexApplicationFile(projectDir);
   if (applicationFile.existsSync()) {
-    return;
+    // This checks for instances of legacy versions of this file. Legacy versions maintained
+    // compatibility with v1 embedding by extending FlutterApplication. If we detect this,
+    // we replace the file with the modern v2 embedding version.
+    if (applicationFile.readAsStringSync().contains('android.app.Application;')) {
+      return;
+    }
   }
   applicationFile.createSync(recursive: true);
 
   final StringBuffer buffer = StringBuffer();
   buffer.write('''
 // Generated file.
+//
 // If you wish to remove Flutter's multidex support, delete this entire file.
+//
+// Modifications to this file should be done in a copy under a different name
+// as this file may be regenerated.
 
 package io.flutter.app;
 
+import android.app.Application;
 import android.content.Context;
 import androidx.annotation.CallSuper;
 import androidx.multidex.MultiDex;
 
 /**
- * Extension of {@link io.flutter.app.FlutterApplication}, adding multidex support.
+ * Extension of {@link android.app.Application}, adding multidex support.
  */
-public class FlutterMultiDexApplication extends FlutterApplication {
+public class FlutterMultiDexApplication extends Application {
   @Override
   @CallSuper
   protected void attachBaseContext(Context base) {

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

@@ -14,7 +14,7 @@ import '../../src/common.dart';
 import '../../src/context.dart';
 
 void main() {
-  testUsingContext('ensureMultidexUtilsExists returns when exists', () async {
+  testUsingContext('ensureMultidexUtilsExists patches file when invalid', () async {
     final Directory directory = globals.fs.currentDirectory;
     final File applicationFile = directory.childDirectory('android')
       .childDirectory('app')
@@ -32,7 +32,33 @@ void main() {
     ensureMultiDexApplicationExists(directory);
 
     // File should remain untouched
-    expect(applicationFile.readAsStringSync(), 'hello');
+    expect(applicationFile.readAsStringSync(), '''
+// Generated file.
+//
+// If you wish to remove Flutter's multidex support, delete this entire file.
+//
+// Modifications to this file should be done in a copy under a different name
+// as this file may be regenerated.
+
+package io.flutter.app;
+
+import android.app.Application;
+import android.content.Context;
+import androidx.annotation.CallSuper;
+import androidx.multidex.MultiDex;
+
+/**
+ * Extension of {@link android.app.Application}, adding multidex support.
+ */
+public class FlutterMultiDexApplication extends Application {
+  @Override
+  @CallSuper
+  protected void attachBaseContext(Context base) {
+    super.attachBaseContext(base);
+    MultiDex.install(this);
+  }
+}
+''');
   }, overrides: <Type, Generator>{
     FileSystem: () => MemoryFileSystem.test(),
     ProcessManager: () => FakeProcessManager.any(),

packages/flutter_tools/test/integration.shard/test_data/multidex_project.dart

@@ -298,18 +298,23 @@ class MultidexProject extends Project {
 
   String get appMultidexApplication => r'''
   // Generated file.
+  //
   // If you wish to remove Flutter's multidex support, delete this entire file.
+  //
+  // Modifications to this file should be done in a copy under a different name
+  // as this file may be regenerated.
 
   package io.flutter.app;
 
+  import android.app.Application;
   import android.content.Context;
   import androidx.annotation.CallSuper;
   import androidx.multidex.MultiDex;
 
   /**
-   * Extension of {@link io.flutter.app.FlutterApplication}, adding multidex support.
+   * Extension of {@link android.app.Application}, adding multidex support.
    */
-  public class FlutterMultiDexApplication extends FlutterApplication {
+  public class FlutterMultiDexApplication extends Application {
     @Override
     @CallSuper
     protected void attachBaseContext(Context base) {