Very basic support for "flutter emulators" which just lists the available Android AVDs.

Relates to:

https://github.com/flutter/flutter/issues/14822
https://github.com/Dart-Code/Dart-Code/issues/490
https://github.com/flutter/flutter/issues/13379

* Allow customization of TextSpan in EditableText

* Addressed PR comments

* Added test with custom-styled EditableText subclass

* More code style fixes

This replaces the --prefer-shared-library flag, which falls back to
regular (non-shared-lib) compile if the NDK is not found, with the
--build-shared-library flag, which exits with an error message if the
NDK is not found.

This simplifies the set of allowed code paths through AOT compile,
resulting in better testability and easier-to-follow logic. It also
results in more predictable behaviour for continuous integration and
other scenarios.

This contains the following commits:

flutter/engine@11d6da3 Restore Skia revision

This contains the following commits:

flutter/engine@2bc3b3e Roll Skia back to d8a189fca3cdfa064de75a280d5af34cc5026bbe
flutter/engine@5d4a38c Roll src/third_party/skia/ d925f2d38..efb84e673 (4 commits)
flutter/engine@61b5028 Read/apply settings that apply process wide before creating any shell components.
flutter/engine@2d60865 Roll src/third_party/skia/ e7df0bb90..d925f2d38 (4 commits)
flutter/engine@8898939 Roll Skia to e7df0bb900ec266021bc6bf4477d533ce1b1f749
flutter/engine@8450a00 Roll Skia back to d8a189fca3cdfa064de75a280d5af34cc5026bbe
flutter/engine@3192223 Roll src/third_party/skia/ 6198f39ad..e7df0bb90 (1 commit)
flutter/engine@5f08fca Roll src/third_party/skia/ cea8e3d13..6198f39ad (1 commit)
flutter/engine@a0be877 Make thread names more descriptive.
flutter/engine@9110e89 Expose View:OfferServiceProvider to UI thread components in a thread safe manner.
flutter/engine@eb5bbda Specify a custom isolate snapshot per AOT shell launch.
flutter/engine@d97b6d8 Allow explicit specification of the isolate snapshot.

We previously blocked iOS AOT builds, but Android debug builds are
always JIT builds.

Updates the message emitted when a kernel compile is skipped in the
build bundle action. Since we now use fingerprinting to enable
performance of script snapshots, AOT snapshots, and kernel compiles,
this helps a bit with debugging.

Also switches to trace level logging.

Minor typo fix for BottomAppBar doc

Add to the extension thrown if it fails instead.

Without this, flutter-tester quits immediately after executing main().

Some APIs (such as the ones in package:test) assume you're running
in a child zone of the test zone by attempting to extract information
from the zone values. When we run runAsync() in the root zone, those
zone values are lost, and such API methods don't work.

The solution is to run in a child zone, but with a specification that
says to use the Root zone for task scheduling (both timers and
microtakss).

Eliminates the restrictions to build in arm64 mode only in iOS project
templates created by flutter create.

Updates all example projects to build as universal binaries that run on
both armv7 and arm64 now that we support both platforms on iOS.

1. Make goldenFileComparator getter return `null` if it's set to the
   uninitialized comparator, which matches the behavior of the setter
   (it sets it to the uninitialized comparator if the caller specifies
   `null`).
2. Make the uninitialized comparator return trivial success (and print
   a message) when asked to compare as opposed to throwing. This ensures
   that the comparator will play nicely with live widget bindings
3. Augment documentation
4. Add assert that test doesn't modify the value of `autoUpdateGoldenFiles`

Build universal armv7+arm64 binary for iOS now that our tooling supports
it.

This change adds support for armv7, arm64, and universal iOS apps.

This change eliminates iOS target architecture hardcoding (previously
arm64 only) and uses the target architecture(s) specified in Xcode's
ARCHS setting ('Architectures' in Xcode Build Settings).

For universal binaries, set ARCHS to its default value, $(ARCHS_STANDARD).

Note that after changing the architecture in Xcode, developers should
run 'pod install' from the ios subdirectory of their project. A separate
change (that will land before this one) will add support for
automatically detecting project file and Podfile changes and re-running
pod install if necessary.

This change also adds an --ios-arch option to flutter build aot. In iOS
AOT builds (in profile and release mode), this dictates which
architectures are built into App.framework. This flag should generally
be unnecessary to set manually since flutter build aot is typically only
invoked internally by flutter itself.

In debug mode iOS builds, we build a stub App.framework with no
functionality, since the engine itself loads the code from the included
bundle and subsequently via hot reload. This is simply done for
consistency with profile/release AOT app structure.

To satisfy the linker, ensure that we're building this code for all
build architectures, not just CURRENT_ARCH.

This is pre-work for supporting arbitrary iOS architectures (armv7,
arm64, universal) in Flutter.

This re-lands feb16d8d, which was reverted due to a flaky test.

This test fails consistently on mac2 and mac3 with the attached Moto G4
devices but passes consistently on other machines.

Adding a delay of 1s right after driver.connect() in setUpAll() causes
it to pass on the machines in question, which suggests a race condition.
Specifically it looks like connect returns the moment Flutter Driver
identifies that the isolate is up and running, but empirically it looks
like we start running the first test before the UI is actually up. This
triggers a failure wherein we start looking for elements before they're
onstage.

Link to viewport.dart:213 at HEAD:
https://github.com/flutter/flutter/blob/b2b46659262c66ff13abc2b8016a94a47646eaad/packages/flutter/lib/src/widgets/viewport.dart#L213

Stack trace:
FlutterDriver waitFor should find text "present"

```
  DriverError: Error in Flutter application: Uncaught extension error while executing waitFor: NoSuchMethodError: The getter 'visible' was called on null.
  Receiver: null
  Tried calling: visible
  #0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:46:5)
  #1      _ViewportElement.debugVisitOnstageChildren. (package:flutter/src/widgets/viewport.dart:213:36)
  #2      WhereIterator.moveNext (dart:_internal/iterable.dart:439:11)
  #3      Iterable.forEach (dart:core/iterable.dart)
  #4      _ViewportElement.debugVisitOnstageChildren (package:flutter/src/widgets/viewport.dart:214:8)
  #5      _DepthFirstChildIterator._reverseChildrenOf (package:flutter_test/src/all_elements.dart:54:15)
  #6      _DepthFirstChildIterator.moveNext (package:flutter_test/src/all_elements.dart:45:19)
  #7      CachingIterable._fillNext (package:flutter/src/foundation/basic_types.dart:252:27)
  #8      _LazyListIterator.moveNext (package:flutter/src/foundation/basic_types.dart:279:21)
  #9      WhereIterator.moveNext (dart:_internal/iterable.dart:438:22)
  #10     CachingIterable._fillNext (package:flutter/src/foundation/basic_types.dart:252:27)
  #11     _LazyListIterator.moveNext (package:flutter/src/foundation/basic_types.dart:279:21)
  #12     Iterable.isEmpty (dart:core/iterable.dart:449:33)
  #13     Iterable.isNotEmpty (dart:core/iterable.dart:456:27)
  #14     FlutterDriverExtension._waitForElement. (package:flutter_driver/src/extension/extension.dart:215:51)
  #15     FlutterDriverExtension._waitUntilFrame (package:flutter_driver/src/extension/extension.dart:197:19)
  #16     FlutterDriverExtension._waitForElement (package:flutter_driver/src/extension/extension.dart:215:11)

  #17     FlutterDriverExtension._waitFor (package:flutter_driver/src/extension/extension.dart:286:11)

  #18     FlutterDriverExtension.call (package:flutter_driver/src/extension/extension.dart:168:51)

  #19     BindingBase.registerServiceExtension. (package:flutter/src/foundation/binding.dart:370:32)
```

Removes a previous hack that no longer appears to help (adding a 1
second delay in setUpAll() does seem to work around this issue though).

* Revert "Build App.framework for all requested architectures (#17296)"

This reverts commit feb16d8d.

This reverts commit f37b84f0.