android:usesCleartextTraffic="true" to <application />也别忘了带上INTERNET允许:
<uses-permission android:name="android.permission.INTERNET" /> <!-- This Line -->
<application
android:name="io.flutter.app.FlutterApplication"
android:label="receipt"
android:usesCleartextTraffic="true" <!-- This Line -->
android:icon="@mipmap/ic_launcher">