* Send RPC request to switch assets directory on hot reload.

This is needed to pick up updated assets that are expected to be picked up on hot reload.

* Assert assets directory is not null.

* Better multiple future wait

* Add type annotation

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

* tweak the text for the 'elements didnt reload' message

* review comments

* prefix items with a list char

* add a hostIsIde param instead of the isDaemonMode top-level function

* add a trailing comma

Various improvements (in particular a new painting fuzzer) to the text manual test.

Some additional documentation.

A fix to Stack to remove an LTR bias: make unpositioned children apply "alignment".

Some more debugging information on RichText and Text.

A fix to the flutter tool to not crash when an RPC call throws an exception.

Reapply "When parts of the program are changed in a hot reload, but not executed during the reassemble, warn that a restart may be needed." (#12490)

This reverts commit 5e7bcbac.

`flutter run --benchmark` was triggering a different quick bailout path in the VM than `flutter run`. The failure has been fixed upstream.

Revert "When parts of the program are changed in a hot reload, but not executed during the reassemble, warn that a restart may be needed. (#12304)" (#12406)

This reverts commit 90028813.

This change caused a few bots to fail with 'JSON-RPC error 110: Extension error', which is odd because _getUnusedChangesInLastReload is not an extension.

When parts of the program are changed in a hot reload, but not executed during the reassemble, warn that a restart may be needed. (#12304)

* Semantic hit test order for Slivers

* ++

* fix double space

* refactoring

* fix test

* comment fix

* tests

* remove accidental debugDumpSemanticsTree

* review comments

* fix button

Bare bones widget inspector support.

Toggle the widget inspector from the flutter tool by pressing 'i'.
When the widget inspector is select mode:
Pointer down to to inspect a widget.
Pointer click to finalize selection of a widget. You can now interact
with the application as you normally would but with the inspected widget
highlighted.
Click the inspect icon in bottom left corner of screen to reactivate
select mode.

* remove the flutter view cache; fix the ability to quit the app after a full restart

* improve test

* update test

Changes introduced in
https://github.com/flutter/engine/commit/8ba522eeae35d8c70ada3c7b8e200ca2274f4f95
has removed from the `_flutter.listViews` the thread synchronization
side effect used during benchmarks.

The thread synchronization is restored via the new
`_flutter.flushUIThreadTasks` RPC.

Fixes https://github.com/flutter/flutter/issues/11241

Related https://github.com/flutter/engine/pull/3898

In
https://github.com/dart-lang/sdk/commit/df8bf384eb815cf38450cb50a0f4b62230fba217
a new functionality of the Dart VM Service Protocol has been introduced.

Clients connected to the Service Protocol are now able to expose
services that other clients (e.g. Observatory) can invoke through the
Service Protocol itself.

With these changes Flutter Tools register them self as a `reloadSources`
(a.k.a. HotReload) capable client.
Observatory is already listening for the clients which expose this
functionality and uses by default the service based version of
`reloadSources` when available, so requesting a HotReload from
Observatory will trigger the full Flutter HotReload.

Related https://github.com/dart-lang/sdk/issues/30023
Related https://github.com/flutter/flutter/pull/11229
Related https://github.com/flutter/flutter/pull/11256

This reverts commit 35c49897.

* Register tools as a reloadSources service

* Fix code style and comments

* Fix nits and Add documentation

* Format error message

* fix an issues with reloading when paused at a breakpoint

* update docs

There is a race where we could complete a future based on a stream event
and then before we cancel the stream subscription, we get another event
and try and complete the future again.

- [x] Switch the reassemble timeout to 5 seconds.
- [x] Print a status message if reassemble fails:

```
Performing hot reload...
Reassembling app.flx$main took too long. Hot reload may have failed.
Reloaded 0 of 418 libraries in 5,322ms.
```

Fixes #9316
Fixes #8861
Fixes #8857
Fixes #8856

- [x] Refresh FlutterViews before issuing a hot reload / restart

Fixes #7248

- [x] Resume the isolate before performing a hot restart.

Fixes #8923

Fixes https://github.com/flutter/flutter/issues/6849

* add @required when there's an assert not null

* address review comments