Redo of https://github.com/flutter/flutter/pull/130728 - code is the same as before. That PR was stuck in Google testing and then I messed up the rebase so started over.

----

Starting in Xcode 15, the simulator is no longer included in Xcode and must be downloaded and installed separately. This adds a validation to `flutter doctor` to warn when the needed simulator runtime is missing.

Validation message looks like:
```
[!] Xcode - develop for iOS and macOS (Xcode 15.0)
    ! iOS 17.0 Simulator not installed; this may be necessary for iOS and macOS development.
      To download and install the platform, open Xcode, select Xcode > Settings > Platforms,
      and click the GET button for the required platform.

      For more information, please visit:
        https://developer.apple.com/documentation/xcode/installing-additional-simulator-runtimes
```

It may also show an error like this when something goes wrong when checking for the simulator:
```
[!] Xcode - develop for iOS and macOS (Xcode 15.0)
    ✗ Unable to find the iPhone Simulator SDK.
```

Note: I'm unsure of in the future if the SDK and the simulator runtime will need to match the exact version or just the major. For now, it only checks against the major version.

Part 3 of https://github.com/flutter/flutter/issues/129558.

PLUS: clean-up of all the unreachable stuff.

Revert "[flutter_tools] Remove sound null safety flag (#123031)"

[flutter_tools] Remove sound null safety flag #2

This reverts commit 7c3088cf.

[flutter_tools] Remove sound null safety flag

This reverts commit 275ab9c6.

Revert "Reland "Remove references to Observatory (#118577)""

This reverts commit 298d8c76.

This reverts commit 2df140f4.

Observatory is being deprecated for Dart 3.0 so it should no longer be
referenced in tooling messaging / flags.

See https://github.com/dart-lang/sdk/issues/50233

* add debugging options to simulator, test more debugging flags, add tests for other launch arguements

* refactor iOS launch arguments to use one function for both simulator and physical devices

* treat dart flags differently between physical and simulator

* Simplify some flags between devices.

Change --disable-service-auth-codes to not always be included for physical devices, only if disableServiceAuthCodes is true.
Change --disable-observatory-publication to be used for simulator devices too.
Change --enable-checked-mode & --verify-entry-points to be used if debuggingEnabled is true regardless of device type.
Chnage --trace-startup to be used for simulator devices too.

* fix ios release mode with buildable app startApp test

* determine observatory-port from deviceVmServicePort and hostVmServicePort

* add comments and remove hasObservatoryPort

* Update flutter_tools to look for new VM service message

The Dart SDK will soon move away from the current Observatory message:

"Observatory listening on ..."

To a new message that no longer references Observatory:

"Dart VM Service listening on ..."

This change updates all tests with mocks to check for the new message
and also adds support for the new message in ProtocolDiscovery.

See https://github.com/dart-lang/sdk/issues/46756

* Fix some parsing locations

* Fix analysis failures

* Update message

* Remove extra comment

* Update message

* Add globals prefix

* Add a new PrebuiltFlutterApplicationPackage interface.

* Review feedback

* Rename bundleDir to uncompressedBundle

This reverts commit 8b6baae4.