Reland #22505, now that flutter/flutter#70637 has landed.

Removes references to dart:ui.Window, and switch usages to PlatformDispatcher or SingletonFlutterWindow, as appropriate.

Embedders that have access to the Dart native API (only Fuchsia now) may perform
library setup in the isolate create callback. The engine used to depend on the
fact the root isolate entrypoint is invoked in the next iteration of message
loop (via the `_startIsolate` trampoline in `isolate_patch.dart`) to ensure that
library setup occur before the main entrypoint was invoked. However, due to
differences in the way in which message loops are setup in Fuchsia, this
entrypoint was run before the callback could be executed. Dart code on Fuchsia
also has the ability to access the underlying event loops directly. This patch
moves the invocation of the create callback to before user dart code has a
chance to run. This difference in behavior on Fuchsia became an issue when the
isolate initialization was reworked in https://github.com/flutter/engine/pull/21820
for null-safety.

Another issue was discovered in that the callback was being invoked twice, I
fixed that too and added a test.

Fixes https://github.com/flutter/flutter/issues/68732

Snapshots compiled with sound null-safety enabled require changes to the way in
which isolates are launched. Specifically, the `Dart_IsolateFlags::null_safety`
field needs to be known upfront. The value of this field can only be determined
once the kernel snapshot is available. This poses a problem in the engine
because the engine used to launch the isolate at shell initialization and only
need the kernel mappings later at isolate launch (when transitioning the root
isolate to the `DartIsolate::Phase::Running` phase). This patch delays launch of
the isolate on the UI task runner till a kernel mapping is available. The side
effects of this delay (callers no longer having access to the non-running
isolate handle) have been addressed in this patch. The DartIsolate API has also
been amended to hide the method that could return a non-running isolate to the
caller.  Instead, it has been replaced with a method that requires a valid
isolate configuration that returns a running root isolate. The isolate will be
launched by asking the isolate configuration for its null-safety
characteristics.

A side effect of enabling null-safety is that Dart APIs that work with legacy
types will now terminate the process if used with an isolate that has sound
null-safety enabled. These APIs may no longer be used in the engine. This
primarily affects the Dart Convertors in Tonic that convert certain C++ objects
into the Dart counterparts. All known Dart Converters have been updated to
convert C++ objects to non-nullable Dart types inferred using type traits of the
corresponding C++ object. The few spots in the engine that used the old Dart
APIs directly have been manually updated. To ensure that no usage of the legacy
APIs remain in the engine (as these would cause runtime process terminations),
the legacy APIs were prefixed with the `DART_LEGACY_API` macro and the macro
defined to `[[deprecated]]` in all engine translation units. While the engine
now primarily works with non-nullable Dart types, callers can still use
`Dart_TypeToNonNullableType` to acquire nullable types for use directly or with
Tonic. One use case that is not addressed with the Tonic Dart Convertors is the
creation of non-nullable lists of nullable types. This hasn’t come up so far in
the engine.

A minor related change is reworking tonic to define a single library target.
This allows the various tonic subsystems to depend on one another. Primarily,
this is used to make the Dart convertors use the logging utilities. This now
allows errors to be more descriptive as the presence of error handles is caught
(and logged) earlier.

Fixes https://github.com/flutter/flutter/issues/59879

This reverts commit dc848f15.

* Add flag to not publish the observatory port over mDNS

* Review edits

* Format

We currently use a mix of C standard includes (e.g. limits.h) and their
C++ variants (e.g. climits). This migrates to a consistent style for all
cases where the C++ variants are acceptable, but leaves the C
equivalents in place where they are required, such as in the embedder
API and other headers that may be used from C.

This reverts commit 4de62c7c.

This reverts commit d16ba48e.

For https://github.com/flutter/flutter/issues/39526

The engine was using a global to store a timestamp representing the
launch of the engine.  This timestamp is initialized with a JNI call
on Android and during shell setup on other platforms.  Later the
timestamp is added to a FlutterEngineMainEnter timeline event used to
measure engine startup time in benchmarks.

This PR removes the global and the JNI call and moves the timestamp
into the settings object.

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

For https://github.com/flutter/flutter/issues/40686

Unit tests added:
- CacheSkSLWorks
- VisitFilesCanBeCalledTwice
- CanListFilesRecursively

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

Adds --observatory-host, which allows callers to specify the localhost
IP binding directly. Allows users to set 0.0.0.0 or the host IP address
as is possible in the standalone VM, rather than restricting users to
loopback.

We retain the default behaviour of binding to loopback since the vast
majority of observatory use-cases involve local access (e.g. host tests
on flutter_tester) or port-forwarded local access (e.g. flutter driver
device tests). However, some scenarios, such as QA test labs, may
benefit from binding to a publicly-accessible IP address.

Using it, a Flutter app can monitor missing frames in the release mode, and a custom Flutter runner (e.g., Fuchsia) can add a custom FrameRasterizedCallback.

Related issues:
https://github.com/flutter/flutter/issues/26154
https://github.com/flutter/flutter/issues/31444
https://github.com/flutter/flutter/issues/32447

Need review as soon as possible so we can merge this before the end of May to catch the milestone.

Tests added:
* NoNeedToReportTimingsByDefault
* NeedsReportTimingsIsSetWithCallback
* ReportTimingsIsCalled
* FrameRasterizedCallbackIsCalled
* FrameTimingSetsAndGetsProperly
* onReportTimings preserves callback zone
* FrameTiming.toString has the correct format

This will need a manual engine roll as the TestWindow defined in the framework needs to implement onReportTimings.

This reverts commit 4ead9c8f.

Reland tracked in https://github.com/flutter/flutter/issues/31433.

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 

The callbacks can be wired in via the Settings object. Both runtime and shell unit-tests have been patched to test this.

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.

This was used in some embedder implementations. But that was a while ago.

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.

This reverts commit 800ea0a4.

This reverts commit b59c4436.