Linux docs_test is 2.01% flaky

[flutter-dashboard]

The post-submit test builder Linux docs_test had a flaky ratio 2.01% for the past 15 days, which is above our 2.00% threshold.

One recent flaky example for a same commit: *
Commit: flutter@016ad85
Failed build:

https://ci.chromium.org/ui/p/flutter/builders/prod/Linux docs_test/1055
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux docs_test/1079
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux docs_test/1134
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux docs_test/1211

Succeeded build:

https://ci.chromium.org/ui/p/flutter/builders/prod/Linux docs_test/1054
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux docs_test/1056
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux docs_test/1057

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).