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.

This allows for testing with MemoryFileSystem, when tests are added.

This simplifies tests by allowing the test author to express the outputs
they'd like the snapshotter to generate without creating a whole new
instance and context with the additional overrides.

Also sets the default set of outputs to none, setting them instead in
the setUp function for script snapshotting.

Makes for easier mocking, particularly for testing AOT snapshot build.

Changes since last engine roll:
```
cc72fc1c3 Roll src/third_party/skia/ ec4e7358b..77e95f706 (1 commit) (#5088)
ee2532f2d Roll src/third_party/skia/ e1c37a7b8..ec4e7358b (1 commit) (#5087)
dd34fd178 Roll src/third_party/skia/ 058f2e6ed..e1c37a7b8 (4 commits) (#5086)
b08b15290 Revert "Roll dart to 29d6e963702fcbb0ed0d5ca67d6b5f977ac0c328 (#5083)" (#5085)
```

* Revert "New Flutter Gallery UI (#16936)"

This reverts commit 7038597b.

A new front-end for the Flutter Gallery example.

As suggested by: https://github.com/lemurheavy/coveralls-public/issues/1103

flutter/engine@9859af1 Roll dart to 29d6e963702fcbb0ed0d5ca67d6b5f977ac0c328

flutter/engine@6a93ef1 Call SkGraphics::Init earlier on Fuchsia.
flutter/engine@f1234eb Add disabled failing test for floating point errors on matrix decomposition.
flutter/engine@ac68263 Hold a global reference to the AssetManager Java object backing the APKAssetProvider
flutter/engine@0e95bb2 Roll src/third_party/skia/ 2d2c1e32c..058f2e6ed (35 commits; 1 trivial rolls)
flutter/engine@3e20562 Roll src/third_party/skia/ 5209d7fce..2d2c1e32c (42 commits; 1 trivial rolls)
flutter/engine@a20fd83 [fuchsia] Remove dead package file generation from core snapshots.
flutter/engine@79b9ffb Fixup Fuchsia content handler post shell refactor.

DoctorValidatorsProvider is injected into Doctor to allow
overriding of DoctorValidators without needing to override
the whole Doctor instance.

* Set text direction in semantics along with Directionality

* update tests for the extra SemanticsNode

CompilerOutput values should never be mutated. Also includes minor
formatting change to the compile() method signature in line with the
style guide.

Minor housekeeping while I'm touching other bits of nearby code.

Pre-factoring for addition of AOT snapshotting unit tests, which emit a
different set of expected outputs.

Wraps the compile function in a class injected via the global context,
which makes it easier to mock in unit tests -- specifically tests for
AOT snapshotting, which already require pretty significant amounts of
mock inputs.