Ensure `android.enableR8` is appended to a new line (#43187)

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

@@ -387,8 +387,14 @@ void migrateToR8(Directory directory) {
   }
   printTrace('set `android.enableR8=true` in gradle.properties');
   try {
-    gradleProperties
-      .writeAsStringSync('android.enableR8=true\n', mode: FileMode.append);
+    // Add `android.enableR8=true` to the next line in gradle.properties.
+    if (propertiesContent.isNotEmpty && !propertiesContent.endsWith('\n')) {
+      gradleProperties
+        .writeAsStringSync('\nandroid.enableR8=true\n', mode: FileMode.append);
+    } else {
+      gradleProperties
+        .writeAsStringSync('android.enableR8=true\n', mode: FileMode.append);
+    }
   } on FileSystemException {
     throwToolExit(
       'The tool failed to add `android.enableR8=true` to ${gradleProperties.path}. '

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

@@ -1410,6 +1410,27 @@ Exception in thread "main" java.lang.NullPointerException
       FileSystem: () => memoryFileSystem,
       ProcessManager: () => FakeProcessManager(<FakeCommand>[]),
     });
+
+    testUsingContext('appends android.enableR8=true to the new line', () {
+      final Directory sampleAppAndroid = fs.directory('/sample-app/android');
+      sampleAppAndroid.createSync(recursive: true);
+      sampleAppAndroid.childFile('gradle.properties')
+        .writeAsStringSync('org.gradle.jvmargs=-Xmx1536M');
+
+      migrateToR8(sampleAppAndroid);
+
+      expect(testLogger.traceText, contains('set `android.enableR8=true` in gradle.properties'));
+      expect(
+        sampleAppAndroid.childFile('gradle.properties').readAsStringSync(),
+        equals(
+          'org.gradle.jvmargs=-Xmx1536M\n'
+          'android.enableR8=true\n'
+        ),
+      );
+    }, overrides: <Type, Generator>{
+      FileSystem: () => memoryFileSystem,
+      ProcessManager: () => FakeProcessManager(<FakeCommand>[]),
+    });
   });
 
   group('isAppUsingAndroidX', () {