Linux flutter_gallery__start_up is 2.56% flaky

[flutterflakybot]

The post-submit test builder Linux flutter_gallery__start_up had a flaky ratio 2.56% 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/Linux%20flutter_gallery__start_up/2166
Commit: flutter@c451b6b
Flaky builds:
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20flutter_gallery__start_up/2166
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20flutter_gallery__start_up/2102

Succeeded builds (3 most recent):
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20flutter_gallery__start_up/2174
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20flutter_gallery__start_up/2173
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20flutter_gallery__start_up/2172

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).