[flutter_tools] Avoid duplicated calls to l10n generation

[perqin]

This issue is concealed by incorrectly written test cases. verify should not be called until previous expect has finished, because the mock function may be called later after the expectation. This seems to be a common pitfall that should be avoided when testing asynchronous code, but isn't mentioned in Dart/Flutter's documentation.

This PR should close the issue #76065.
Fixes #76065

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. Instead, I fixed the wrong test cases.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[shihaohong]

LGTM. Thanks for reporting the problem, as well as the fix with tests!