Android机当长期放置或者开启应用过多时，会导致flutter引擎中的dart runtime被清理，导致页面空白

## Steps to Reproduce

1. 直接打开软件，从原生页面跳转到flutter页面（可能触发）
2. 打开软件，再打开其它软件把手机内存占满，再打开软件极有可能触发

**Flutter Boost Version** 1.12.13.hotfix9
**Target Platform:** Android
**Target OS version/browser:** Android5.1（API22）
**Devices:** ZTEBA611C

## 猜测原因

目前debug下来看，出现这种情况下，dart 的runtime已经不在内存中了（猜测是被系统清理了），不过Android端的flutterEngine还正常在内存中，并且其中flutterJNI及dartExecutor都正常，所以无法感知dart runtime不在内存中，导致了页面空白

## Logs
add your crash log or something else.

```
[✓] Flutter (Channel v1.12.13-hotfixes, v1.12.13+hotfix.9, on Mac OS X 10.15.4 19E287, locale zh-Hans-CN)
• Flutter version 1.12.13+hotfix.9 at /Users/admin/Desktop/Application/flutter
• Framework revision f139b11009 (5 months ago), 2020-03-30 13:57:30 -0700
• Engine revision af51afceb8
• Dart version 2.7.2

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
• Android SDK at /usr/local/Caskroom/android-sdk/4333796
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-29, build-tools 29.0.3
• ANDROID_HOME = /usr/local/share/android-sdk
• Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.7)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 11.7, Build version 11E801a
• CocoaPods version 1.9.1

[✓] Chrome - develop for the web
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 3.5)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin version 42.1.1
• Dart plugin version 191.8593
• Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] VS Code (version 1.48.2)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 3.14.0

[✓] Connected device (4 available)
• ZTE BA611C • ZTEBA611C • android-arm64 • Android 5.1 (API 22)
• iPhone XR • 6CCD6563-7474-4687-9335-78A2A503D264 • ios • com.apple.CoreSimulator.SimRuntime.iOS-12-0 (simulator)
• Chrome • chrome • web-javascript • Google Chrome 85.0.4183.83
• Web Server • web-server • web-javascript • Flutter Tools

• No issues found!
```

No source file or test is named. Start by reproducing the blank Flutter page on the listed Android 5.1 device using the memory-pressure steps, then inspect the Android FlutterEngine and Dart runtime lifecycle described in the report. Done means the runtime-loss case is detected and the page no longer remains blank.

Written by the indexing model from the issue text.