不想执行 main 函数，而是想把入口函数换成别的， 方法如下：

Android:

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.util.Log;

// 导入 flutter 相关的类
import io.flutter.embedding.android.FlutterActivity;
import io.flutter.embedding.engine.FlutterEngineGroup;
import io.flutter.embedding.engine.FlutterEngine;
import io.flutter.embedding.engine.dart.DartExecutor.DartEntrypoint;
import io.flutter.embedding.engine.loader.FlutterLoader;
import io.flutter.FlutterInjector;

public class MainActivity extends Activity {
    FlutterEngine flutterEngine;

    @Override
    protected void onCreate(Bundle bundle) {
        super.onCreate(bundle);
        runMain2();
    }

    void runMain2() {
        Log.d("main", "runMain2()");
        Context context = this.getBaseContext();
        FlutterEngineGroup group = new FlutterEngineGroup(context);
        FlutterLoader flutterLoader = FlutterInjector.instance().flutterLoader();
//        DartEntrypoint entrypoint = new DartEntrypoint(flutterLoader.findAppBundlePath(), "main2");
        DartEntrypoint entrypoint = new DartEntrypoint(null, "main2");
        flutterEngine = group.createAndRunEngine(context, entrypoint);
    }
}

iOS：

  FlutterEngineGroup *group = [[FlutterEngineGroup alloc] initWithName:@"main" project:nil];
  FlutterEngine *engine = [group makeEngineWithEntrypoint:@"main2" libraryURI:nil];
  [engine run];