Add test coverage around startup race and missing file on startup.

[davidmorgan]

For #2170, start adding test coverage.

I initially started adding test coverage around symlinks, but encountered two existing issues that made it awkward

• there is a race on startup on MacOS that means there might be events from before the watch; the existing tests are flaky because of this issue
• the file watchers have inconsistent behavior if the file watched does not exist

Cover both of these with tests before making any changes :)

Add expectNoEvents and use it in one existing test as it's more readable than triggering then waiting for a different event.

[gemini-code-assist]

Code Review

This pull request adds valuable test coverage for a startup race condition and for watching non-existent files. The refactoring of shared tests and the addition of the expectNoEvents helper are good improvements. However, I've identified a couple of critical issues in the new tests. The test for the startup race condition is not functioning as intended due to misuse of global test helpers, and another test has an incorrect expect call. Addressing these issues will make this a strong contribution to the test suite's reliability.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
watcher	None	1.1.3	1.1.3	1.1.3	✔️

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/bazel_worker/benchmark/benchmark.dart
pkgs/bazel_worker/example/client.dart
pkgs/bazel_worker/example/worker.dart
pkgs/benchmark_harness/integration_test/perf_benchmark_test.dart
pkgs/boolean_selector/example/example.dart
pkgs/clock/lib/clock.dart
pkgs/clock/lib/src/clock.dart
pkgs/clock/lib/src/default.dart
pkgs/clock/lib/src/stopwatch.dart
pkgs/clock/lib/src/utils.dart
pkgs/clock/test/clock_test.dart
pkgs/clock/test/default_test.dart
pkgs/clock/test/stopwatch_test.dart
pkgs/clock/test/utils.dart
pkgs/coverage/lib/src/coverage_options.dart
pkgs/html/example/main.dart
pkgs/html/lib/dom.dart
pkgs/html/lib/dom_parsing.dart
pkgs/html/lib/html_escape.dart
pkgs/html/lib/parser.dart
pkgs/html/lib/src/constants.dart
pkgs/html/lib/src/encoding_parser.dart
pkgs/html/lib/src/html_input_stream.dart
pkgs/html/lib/src/list_proxy.dart
pkgs/html/lib/src/query_selector.dart
pkgs/html/lib/src/token.dart
pkgs/html/lib/src/tokenizer.dart
pkgs/html/lib/src/treebuilder.dart
pkgs/html/lib/src/utils.dart
pkgs/html/test/dom_test.dart
pkgs/html/test/parser_feature_test.dart
pkgs/html/test/parser_test.dart
pkgs/html/test/query_selector_test.dart
pkgs/html/test/selectors/level1_baseline_test.dart
pkgs/html/test/selectors/level1_lib.dart
pkgs/html/test/selectors/selectors.dart
pkgs/html/test/support.dart
pkgs/html/test/tokenizer_test.dart
pkgs/html/test/trie_test.dart
pkgs/html/tool/generate_trie.dart
pkgs/pubspec_parse/test/git_uri_test.dart
pkgs/stack_trace/example/example.dart
pkgs/watcher/test/custom_watcher_factory_test.dart
pkgs/yaml_edit/example/example.dart

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[davidmorgan]

Whoops, some of the new tests were not actually wired up to run--because I copied them from the PR where I originally wrote them.

I will fix and resend for review :)

[davidmorgan]

Okay, this looks good now. The test failures are unrelated, pre-existing flakiness: they're on directory_watcher, this PR is just on file_watcher.