* add trailing commas on list/map/parameters
* add trailing commas on Invocation with nb of arg>1
* add commas for widget containing widgets
* add trailing commas if instantiation contains trailing comma
* revert bad change
* Remove many timeouts.
These are essentially self-inflicted race conditions. Instead of timeouts we're going to try a more verbose logging mechanism that points out when things are taking a long time.
* Get the attach tests to pass.
* Apply review comments from Todd
* More review comment fixes
* Put back the extended timeouts here now that I know why we have them...
* Roll engine to b6df7a637498ca9beda1fa9cd7210e3202ea599f.
Changes since last roll:
```
b6df7a637 Roll dart to 290c576264faa096a0b3206c71b2435309d9f904. (#4771)
a6764dbd5 Add sources for Fuchsia target. (#4763)
2d5900615 [fuchsia] Remove unused header file. (#4769)
9717063b7 Revert "Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)" (#4768)
9a9814312 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)
e74e8b35c [async] Update includes of async headers to new path (#4760)
e2c4b2760 Use Dart 2 camel case constants in the engine Dart libraries (#4766)
9c1e48434 Updates for Fuchsia roll. (#4765)
14c940e27 Switch from fxl::Mutex to std::mutex (#4764)
debf82c0b Roll Garnet (#4759)
5bffdefbb Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (#4761)
```
* Reject requests for hot reload if a hot reload is already in progress.
Fixes#14184
* Implement TODO, verifying further hot reloads complete sucessfully.
* Fix year on new file.
* Add missing type annotations to fix lints
* Add run_machine_concurrent_hot_reload to manifest for CI
* Reformat document
... but undo things that cause lints (like single-line ifs)
* Extract std stream transformations
* Make inProgressHotReload private
* Disallow all types of reload while hot reload in progress
* Simplify code handling in-progress hot reloads
