Since this is currently only meant to be used by the embedding internally, the setter in Objective-C is only exposed via the FlutterDartProject private class extension. Unit tests have been added to the shell_unittests harness.

Fixes https://github.com/flutter/flutter/issues/37641

- Removed unused instance variable
- Explicitly listed nullability

libapp.so contains compiled application Dart code.  On most Android systems,
this library can be loaded by calling dlopen("libapp.so"), which will search
Android's default library directories.

On some Android devices this does not work as expected.  As a workaround, this
patch provides a fallback path to libapp.so based on ApplicationInfo.nativeLibraryDir.

Fixes https://github.com/flutter/flutter/issues/35838

This reverts commit bd8c5b13.

Reverts flutter/engine#8656

Reason:  https://github.com/flutter/engine/pull/8656 seems to break the framework windows tests and the engine roll (see https://cirrus-ci.com/task/4704667236827136 and https://github.com/flutter/flutter/pull/31330). The failure has been consistent for 7 consecutive engine-to-framework auto-rolls.

TBR: @chinmaygarde 

If the mapping callback is not set or it the callback returns invalid data, ICU initialization will be embedder responsibility.

This affects all embedders and the following have been audited:
* Android: Via a symbol mapping.
* iOS: Via a file mapping.
* Embedder: Via a file mapping.
* Fuchsia: Via a VMO mapping
* Test shells and Flutter tester: Via file mapping with ICU data needing to be next to the executable.

Some components in the Flutter engine were derived from the forked blink codebase. While the forked components have either been removed or rewritten, the use of the blink namespace has mostly (and inconsistently) remained. This renames the blink namesapce to flutter for consistency. There are no functional changes in this patch.

* Add overloads for lookup that lets you specify a bundle

* check bundle with defaultBundleIdentifier before main bundle

Link dart:* sources into engine for debugger source support

Currently, dart:* libraries appear to have no source in
debuggers like Observatory. With this change, these sources will be
available in debug mode applications. Sources for dart:* libraries are
lazily loaded on a script-by-script basis.

Refer to https://dart-review.googlesource.com/c/sdk/+/93375 for the Dart
SDK change.

Technically both of these are part of the public API exposed in
Flutter.framework. Neither is used within Flutter itself, and both have
been broken since the removal of Dart 1 support, so eliminating rather
than marking unavailable.

These settings were specific to Dart 1 and are no longer used in the
engine. This eliminates them from the Settings class.

* Create FlutterEngine to manage a Shell to support maintaining execution state across instances of FlutterViewControllers
* Refactor PlatformViewIOS to support adding or removing a FlutterViewController dynamically
* Refactor FlutterDartHeadlessCodeRunner to implement FlutterEngine
* Refactor FlutterViewController to accept a FlutterEngine at initialization or to create one for backwards compatibility; any Shell related responsibilities are now passed through to the Engine instance
* Remove FlutterNavigationController (unused)
* Update all public Objective C doc comments to be consistent and formatable
* Add public documentation to all public headers

* Change FlutterDartProject to keep precedence when looking for the assets bundle

* Change assetsPath to fallback to mainBundle when the defined bundle does not contain assets

To make "Engine run configuration was invalid." failures (e.g.
https://github.com/flutter/flutter/issues/20777) a bit easier to
diagnose, add some more logging to `FlutterDartProject` to verify
that snapshot assets were properly bundled.

Following fixes in dart-lang/sdk@91cbb57cd5cd012d5aa66bfb6eb3a1306271f431

This reverts commit e3133e0e.

 - Add missing Dart 2 flags to gen_snapshot invocation.
 - Disable brittle service test.

This reverts commit 4c2448d1.

 - Switch core snapshot to Dart 2 and remove support for loading platform.dill.
 - Remove support for loading script snapshots.
 - Remove support for loading source.
 - Remove settings and fix names to reflect the above.
 - Remove support for loading the service isolate from source.

The following were previously marked as deprecated over a month ago:
* `FlutterStandardBigInteger`
* `-[FlutterDartProject initFromDefaultSourceForConfiguration]`

Remove their implementations and mark them as unavailable.

If `FlutterDartProject` found an `FLTLibraryPath` entry in an iOS
application's `Info.plist`, it assumed that values that were valid
filesystem paths were paths to bundles.  If the attempt to retrieve
the `NSBundle` fails, `FlutterDartProject` ignored the failure and
then would assign `nil` to a C++ `std::string`, resulting in a null
pointer dereference.

Add some failure checks to prevent this.

Switch all embedders to use platform_strong.dill instead of platform.dill (the flutter_tester binary already does this) (#6024)

We'd like the ability to add Flutter to existing iOS applications
without requiring that they set `FLTLibraryPath` and `FLTAssetsPath`
in the main bundle's `Info.plist`.

1. Modify `-[FlutterDartProject initWithPrecompiledDartBundle:]` to
   support setting the library and assets path from the specified
   `NSBundle` instead.

2. If no `NSBundle` is explicitly specified, look for one with a
   default bundle identifier ("io.flutter.flutter.app") before
   falling back to the main NSBundle.

Also remove `+[FlutterDartProject pathForFlutterAssetsFromBundle:]`
because we don't use it internally, and it isn't exposed in the
header file.

* Deprecate -[FlutterDartProject initFromDefaultSourceForConfiguration] (#18886)

`-[FlutterDartProject initFromDefaultSourceForConfiguration]` no
longer seems very useful.  It calls `-initWithPrecompiledDartBundle:`
or `-initWithFlutterAssets:dartMain:packages:`, but since it now
passes `nil` for all arguments, both paths end up doing the same
thing.

Additionally, `-initFromDefaultSourceForConfiguration` is awkward to
use in Swift.  The automatically generated Swift interface is:

    public convenience init!(fromDefaultSourceForConfiguration: ())

and it's not obvious how to call that.

Let's deprecate `-initFromDefaultSourceForConfiguration` and instead
expect callers to use the existing `-init` method. (We can make
`-init` do different things for different build configurations later
if necessary.)

Bonus: Rename some parameters to make it more obvious when they may
be `nil`.

Broke runtime_unittests, which are still running directly from source.

This reverts commit 0ea93c3a.

Eliminates support for running directly from sources or script snapshots. In
debug mode, we run from a kernel snapshot; in profile and release modes, we
link in AOT-compiled code.

Renames --dart-non-checked-mode to --disable-dart-asserts since checked mode
does not make sense in Dart 2.

 - Remove assumption that we're in Dart 2 mode only if there's a platform kernel.
 - Load core snapshots from the package instead of linking them in.

* Re-land "Support multiple shells in a single process. (#4932)"

This reverts commit 723c7d01.

This reverts commit a3327bff.

This reverts commit 9199b40f.