Recover from watcher restart.

[davidmorgan]

For #4154, WIP

[github-actions]

PR Health

Changelog Entry ❗

Package	Changed Files
package:build_runner	build_runner/lib/src/daemon/daemon_builder.dart build_runner/lib/src/entrypoint/watch.dart build_runner/lib/src/generate/watch_impl.dart build_runner/lib/src/watcher/graph_watcher.dart build_runner/lib/src/watcher/node_watcher.dart
package:build_runner_core	build_runner_core/lib/src/generate/build_result.dart

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