* Add a `matchesGoldenFile()` async matcher that will match
  a finder's widget's rasterized image against a golden file.
* Add support for pluggable image comparison backends
* Add a default backend that does simplistic PNG byte
  comparison on locally stored golden files.
* Add support for `flutter test --update-goldens`, which will
  treat the rasterized image bytes produced during the test
  as the new golden bytes and update the golden file accordingly

Still TODO:

* Add support for the `flutter_test_config.dart` test config hook
* Utilize `flutter_test_config.dart` in `packages/flutter/test`
  to install a backend that retrieves golden files from a dedicated
  `flutter/goldens` repo

https://github.com/flutter/flutter/issues/16859

Follow-up to https://github.com/flutter/flutter/pull/17063

https://github.com/flutter/flutter/issues/16859

This reverts commit 6e26cc28.

Moves the kernel compile step to the beginning of the AOT build in a
separate method. This is pre-factoring for iOS universal builds where
the kernel build happens once, but we then snapshot twice: once for
armv7 and once for arm64.

This also writes dependencies to build/kernel_compile.d rather than
build/aot/snapshot.d, since that is immediately overwritten by
gen_snapshot.

This will allow callers to run real async code within a widget test.

https://github.com/flutter/flutter/issues/16859

* Wrap `expectLater()` in flutter_tester so that callers can
  wait for the matcher to complete in their tests

https://github.com/flutter/flutter/issues/16859

* Roll to pre-dart engine.

* Roll to dart roll engine.

https://github.com/flutter/flutter/issues/16859

* Give more room for the front layer drag on iPhone X

* Tweak the animation visual effects

* review lint

This isn't user-specifiable through a flag, so no need to pass it as a
parameter.

Removes unused var.

When forwarding all ports, run a test connection to each Dart VM instance, stopping port forwarding if the connection fails.

Also does the same for any batched calls that hit all Dart VM instances.

This is a simple refactoring with no functional changes. We now reuse
the existing _isBuildRequired() and _writeFingerprint() functions and
share them with script snapshotting rather than reimplementing their
logic.

Changes the signatures of both to support multiple output files (as
required for AOT snapshotting).

Fixes a bug introduced in 82f969ff where
the depfile used for AOT snapshotting, useful in particular for skipping
gen_snapshot when inputs/outputs haven't changed since the last build.

Changes since the last roll:

```
e11905c6e Add tooltip method to Android AccessibilityBridge (#5103)
2ee36e270 Always enable verbose logging on fuchsia. (#5104)
131349f83 Update flutter runner process name to reflect last running shell. (#5102)
ad9826cbb Roll src/third_party/skia/ c79e2d155..47ed6f10e (10 commits; 1 trivial rolls) (#5100)
7617d2889 libtxt: post-process glyph positions in order to accurately right-justify text (#5093)
895c965ce Set file modified callback. (#5096)
956f3eeca Roll src/third_party/skia/ 5c9369eb1..c79e2d155 (7 commits) (#5099)
6e5112214 Roll src/third_party/skia/ 643ab1bf5..5c9369eb1 (1 commit; 1 trivial rolls) (#5098)
a9180ca20 Roll src/third_party/skia/ 6b043cb9c..643ab1bf5 (3 commits; 3 trivial rolls) (#5097)
82d461049 Roll src/third_party/skia/ 1366282bc..6b043cb9c (2 commits) (#5095)
fb4926e65 Roll src/third_party/skia/ 580aee2fa..1366282bc (2 commits) (#5094)
c249b7026 Path metrics/getBounds/combinations again (#4957)
df255b828 Roll src/third_party/skia/ de5cffbc4..580aee2fa (16 commits) (#5092)
e44e1b49f Initial plugin support for background downloads + content fetching (#5082)
78f8a0f45 [fuchsia] Changes for running Dart 2 on flutter_runner. (#5080)
cbec56a3d Roll src/third_party/skia/ 77e95f706..de5cffbc4 (14 commits) (#5090)
8da9398ed Throw an exception if a secondary isolate tries to send a platform message (#5069)
```

Inlines the buildAotSnapshot function, which simply passes through its
arguments to Snapshotter.buildAotSnapshot.

This is required for iOS debug builds, but unused otherwise. In theory,
Android debug builds could be run in this mode, but this is historically
untested and adds unnecessary complexity to the code. If ad-hoc testing
is required, it can be patched in when necessary.

This re-lands 8c4f0c0d with a fix to
xcode_backend.sh to eliminate the use of --interpreter.

Requires a fix to xcode_backend.sh. Will followup momentarily with that
fix.

This reverts commit 8c4f0c0d.

This is required for iOS debug builds, but unused otherwise. In theory,
Android debug builds could be run in this mode, but this is historically
untested and adds unnecessary complexity to the code. If ad-hoc testing
is required, it can be patched in when necessary.

See https://github.com/flutter/flutter/pull/15383

See https://docs.flutter.io/ to see what I mean.

This moves AOT snapshotting out of build_aot.dart and into the
Snapshotter class. It also adds unit tests for iOS debug, profile, and
release builds.

Increased the channels test timeout

Use a space instead of a zero-width space to calculate the preferred height of a line of text (#16972)

The zero-width space character may not be supported by the font requested in
the text style.  If that happens, then libtxt will fall back to another font
to render that character, resulting in text metrics that do not match the
intended font.

Fixes https://github.com/flutter/flutter/issues/16257

This fixes and re-lands 4bb7496b, which
was reverted in ceade39c due to test
failures caused by a bad rebase.

This reverts commit 4bb7496b.

Allows for better mockability when tests are added.