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.

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 reverts commit c64ace84.

This will allow external tools that wrap our test harness to share the
code that generates the test bootstrap.

This change exposed an issue whereby the LocalGoldenFileComparator
was being too strict in its URI handling, so this changes relaxes
that constraint as well (and adds associated tests).

The new values are not ready for use yet inside Google

If the developer changes their Xcode build settings and their project
has plugins, pod install is required, (e.g. to pick up changes to the
target architecture).

Similarly, manual edits to the Podfile should trigger a pod install.

* Roll engine to e976be13c51448f89107d082ec81e2b6731671fa

* move away from deprecated constants

* Exclude flutter_goldens package from dartdoc because it's for internal
  use only
* Document why flutter_tools doesn' tneed to be excluded from the list of
  packages to document
* Performance optimization in the flutter comparator, and associated
  test updates.

Adds a Fingerprinter utility class that can be used to compute unique
fingerprints for a set of input paths and build options, compare to the
output of a previous run, and skip the build action if no inputs or
options have changed. The existing Fingerprint class still does all the
heavy lifting. Fingerprinter adds common operations such as
reading/writing/comparing fingerprints and parsing depfiles.

This migrates existing uses of Fingerprint over to Fingerprinter.

This also adds better fingerprinting to AOT snapshotting, which
previously failed to include several options in its fingerprint
(--preview-dart-2, --prefer-shared-library).

Revert longPress/Tap and Gallery changes

This change adds Dart VM event support (listening for when a VM starts/stops by using a periodic heartbeat).

This also adds support to connect to a specific `IsolateRef` through the flutter driver, so that when an application spawns, it can immediately be driven (as shown in included example code).

The "about" text was apparently too long.