Linux web_tool_tests is 4.02% flaky

[flutterflakybot]

The post-submit test builder Linux web_tool_tests had a flaky ratio 4.02% 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/Linux%20web_tool_tests/2643
Commit: flutter@6f9cefe
Failed build:

https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2650
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2643
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2641
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2639
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2638
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2636
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2607
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2582
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2576

Succeeded build (3 most recent):

https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2697
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2696
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2695

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).