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.

* [fuchsia] Pass a dummy command to the ssh tunnel setup

* Address analyzer lint

* Update fuchsia_remote_debug_protocol

Changes include:

flutter/engine@ec61147 replace nextNodeId/previousNodeId with hitTestPosition
flutter/engine@2970c09 Use the default microtask scheduler on all isolates except the UI isolate
flutter/engine@96af3b2 ios: remove geometric order; it will be supplied by framework
flutter/engine@3b84a47 Apply unaddressed comments in pull/5062
flutter/engine@29816a6 Roll src/third_party/skia/ 3d0e8507f..5209d7fce (14 commits)
flutter/engine@4691a0b Import intrusively ref counted shared pointers into FML.

Fixes https://github.com/flutter/flutter/issues/16496

Renames buildSnapshot to build buildScriptSnapshot in preparation for
AOT snapshot tests.

New a11y traversal:

- sort direct sibling SemanticsNodes only
- use new sorting algorithm
- implement RTL
- test semantics in traversal order by default
- add AppBar traversal test
- breaking: remove nextNodeId/previousNodeId from the framework
- breaking: remove DebugSemanticsDumpOrder.geometricOrder