Repository: flutter
Issue: [web] exceptions on mouse move when window loses focus
Labels: engine,platform-web,platform-linux,a: desktop,a: mouse,P2,team-engine,triaged-engine

On desktop chrome for Linux, running a flutter application in debug mode and then losing window focus causes all subsequent mouse moves to throw the assertion below:

Error:
```
dart_sdk.js:5312 Uncaught Error: Assertion failed: org-dartlang-sdk:///flutter_web_sdk/lib/_engine/engine/pointer_converter.dart:271:18
!state.down
is not true
    at Object.throw_ [as throw] (dart_sdk.js:4021)
    at Object.assertFailed (dart_sdk.js:3969)
    at _engine.PointerDataConverter.new.convert (dart_sdk.js:145428)
    at _engine.PointerAdapter.new.[_convertEventToPointerData] (dart_sdk.js:145120)
    at dart_sdk.js:145088
    at HTMLElement.<anonymous> (dart_sdk.js:144990)
```

Doctor:

```
[✓] Flutter (Channel master, v1.13.1-pre.94, on Linux, locale en_US.UTF-8)
    • Flutter version 1.13.1-pre.94 at /usr/local/google/home/jonahwilliams/Documents/flutter
    • Framework revision 36e599eb5d (12 hours ago), 2019-12-11 07:27:13 +0100
    • Engine revision 12bf95fd49
    • Dart version 2.7.0 (build 2.7.0-dev.2.1 8b8894648f)

[!] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /usr/local/google/home/jonahwilliams/Android/Sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-29, build-tools 29.0.2
    • Java binary at: /opt/android-studio-with-blaze-3.5/jre/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)
    ! Some Android licenses not accepted.  To resolve this, run: flutter doctor --android-licenses

[✓] Chrome - develop for the web
    • Chrome at google-chrome

[✓] Linux toolchain - develop for Linux desktop
    • clang++ 8.0.1
    • GNU Make 4.2.1

[!] Android Studio (version 3.4)
    • Android Studio at /opt/android-studio-with-blaze-3.4
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    ✗ Unable to find bundled Java version.
    • Try updating or re-installing Android Studio.

[!] Android Studio (version 3.3)
    • Android Studio at /usr/local/google/home/jonahwilliams/Documents/android-studio
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1248-b01)

[!] Android Studio (version 3.5)
    • Android Studio at /opt/android-studio-with-blaze-3.5
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] IntelliJ IDEA Community Edition (version 2019.1)
    • IntelliJ at /usr/local/google/home/jonahwilliams/Documents/idea-IC-191.7479.19
    • Flutter plugin version 36.1.4
    • Dart plugin version 191.7830

[!] IntelliJ IDEA Ultimate Edition (version 2018.3)
    • IntelliJ at /opt/intellij-ue-2018.3
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    • Dart plugin version 183.5912.23
    • For information about installing plugins, see
      https://flutter.dev/intellij-setup/#installing-the-plugins

[✓] IntelliJ IDEA Ultimate Edition (version 2019.1)
    • IntelliJ at /opt/intellij-ue-2019.1
    • Flutter plugin version 39.0.2
    • Dart plugin version 191.8423

[!] VS Code (version 1.39.2)
    • VS Code at /usr/share/code
    ✗ Flutter extension not installed; install from
      https://marketplace.visualstudio.com/items?itemName=Dart-Code.flutter

[✓] Connected device (3 available)
    • Linux      • Linux      • linux-x64      • Linux
    • Chrome     • chrome     • web-javascript • Google Chrome 79.0.3945.79
    • Web Server • web-server • web-javascript • Flutter Tools
```