Add Flutter to existing apps，flutter engine leak or doesn't release on iOS platform #27149
Description
➜ ~ flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel master, v1.1.10-pre.193, on Mac OS X 10.14.2 18C54, locale zh-Hans-CN)
[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[✓] iOS toolchain - develop for iOS devices (Xcode 10.1)
[✓] Android Studio (version 3.1)
[✓] Connected device (1 available)
• No issues found!