https://github.com/flutter/flutter/issues/162567
- Uses the `bootstrapScript` field in `loadConfig` to run a script after
all scripts have loaded.
- This script just calls a callback that is set up beforehand and calls
main.
- Modifies the callback that calls `dartDevEmbedder.runMain` to wait
until both DWDS called main and all scripts have loaded.
- Unskips hot reload tests now that the race condition should no longer
exist.
## 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 signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.
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