Windows web_tool_tests is 5.56% flaky

[flutterflakybot]

The post-submit test builder Windows web_tool_tests had a flaky ratio 5.56% for the past 15 days, which is above our 2.00% threshold.

One recent flaky example for a same commit: https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2501
Commit: flutter@9bd6e21
Failed build:

https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2501
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2489
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2481
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2480
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2474
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2457
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2448
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2443
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2440
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2438

Succeeded build (3 most recent):

https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2505
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2504
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20web_tool_tests/2503

Please follow https://github.com/flutter/flutter/wiki/Reducing-Test-Flakiness#fixing-flaky-tests to fix the flakiness and enable the test back after validating the fix (internal dashboard to validate: go/flutter_test_flakiness).