On Fuchsia, add a build flag for compositing OpacityLayers using the system
compositor vs Skia, which exposes a fastpath for opacity via Scenic.
This will only work under certain circumstances, in particular nested
OpacityLayers will not render correctly!

On Fuchsia, add a build flag for compositing PhysicalShapeLayers using
the system compositor vs Skia. Set to off by default, which restores
performant shadows on Fuchsia.

Remove the opacity exposed from ChildView, as that was added mistakenly.

Finally, we centralize the logic for switching between the
system-composited and in-process-composited paths inside of
ContainerLayer. We also centralize the logic for computing elevation
there. This allows the removal of many OS_FUCHSIA-specific code-paths.

Test: Ran workstation on Fuchsia; benchmarked before and after
Bug: 23711
Bug: 24163

* Fix broken tests

Account for root surface transformation on the surfaces managed by the external view embedder. (#11384)

The earlier design speculated that embedders could affect the same
transformations on the layers post engine compositor presentation but before
final composition.

However, the linked issue points out that this design is not suitable for use
with hardware overlay planes. When rendering to the same, to affect the
transformation before composition, embedders would have to render to an
off-screen render target and then apply the transformation before presentation.
This patch negates the need for that off-screen render pass.

To be clear, the previous architecture is still fully viable. Embedders still
have full control over layer transformations before composition. This is an
optimization for the hardware overlay planes use-case.

Fixes b/139758641

Added channel buffers to 'ui' so that messages have a place to
go until message handling is setup.

This reverts commit e97ed36e.

Reason for revert: Stream is considered a bad API: https://github.com/flutter/engine/pull/11041#issuecomment-526892247

This was required by the C++ spec pre-C++11 and still gets picked up in
the Fuchsia tree due to -Wnewline-eof. It fixes:

    ../../third_party/flutter/lib/ui/text/line_metrics.h:75:47: error: no newline at end of file [-Werror,-Wnewline-eof]
    #endif  // FLUTTER_LIB_UI_TEXT_LINE_METRICS_H_

We will end up creating fewer threads in tests.

Without this, developers have to override `onReportTimings` to listen for `FrameTiming`.
That can potentially break previous `onReportTimings` listeners if they forget to call
the old listener in their new callback.

This PR replaces the similar RP in the framework: https://github.com/flutter/flutter/pull/38574

Once this PR landed, we'll have to create another framework PR to use the stream to replace
`onReportTimings` usages.

Once that's done, we can then propose the breaking change of removing the deprecated
`onReportTimings`.

* Pass Android Q system gesture inset information from insets.systemGestureInsets to Window.systemGestureInsets

ParagraphImpl was used to switch between libtxt and Blink implementations
of text rendering and is now obsolete.

This patch allows embedders to split the Flutter layer tree into multiple
chunks. These chunks are meant to be composed one on top of another. This gives
embedders a chance to interleave their own contents between these chunks.

The Flutter embedder API already provides hooks for the specification of
textures for the Flutter engine to compose within its own hierarchy (for camera
feeds, video, etc..). However, not all embedders can render the contents of such
sources into textures the Flutter engine can accept. Moreover, this composition
model may have overheads that are non-trivial for certain use cases. In such
cases, the embedder may choose to specify multiple render target for Flutter to
render into instead of just one.

The use of this API allows embedders to perform composition very similar to the
iOS embedder. This composition model is used on that platform for the embedding
of UIKit view such and web view and map views within the Flutter hierarchy.
However, do note that iOS also has threading configurations that are currently
not available to custom embedders.

The embedder API updates in this patch are ABI stable and existing embedders
will continue to work are normal. For embedders that want to enable this
composition mode, the API is designed to make it easy to opt into the same in an
incremental manner.

Rendering of contents into the “root” rendering surface remains unchanged.
However, now the application can push “platform views” via a scene builder.
These platform views need to handled by a FlutterCompositor specified in a new
field at the end of the FlutterProjectArgs struct.

When a new platform view in introduced within the layer tree, the compositor
will ask the embedder to create a new render target for that platform view.
Render targets can currently be OpenGL framebuffers, OpenGL textures or software
buffers. The type of the render target returned by the embedder must be
compatible with the root render surface. That is, if the root render surface is
an OpenGL framebuffer, the render target for each platform view must either be a
texture or a framebuffer in the same OpenGL context. New render target types as
well as root renderers for newer APIs like Metal & Vulkan can and will be added
in the future. The addition of these APIs will be done in an ABI & API stable
manner.

As Flutter renders frames, it gives the embedder a callback with information
about the position of the various platform views in the effective hierarchy.
The embedder is then meant to put the contents of the render targets that it
setup and had previously given to the engine onto the screen (of course
interleaving the contents of the platform views).

Unit-tests have been added that test not only the structure and properties of
layer hierarchy given to the compositor, but also the contents of the texels
rendered by a test compositor using both the OpenGL and software rendering
backends.

Fixes b/132812775
Fixes flutter/flutter#35410

SingleFrameCodec had been holding a persistent handle to the peer Dart object
while waiting for the decoder callback.  However, Dart code could call
Codec.dispose during that interval, which would cause the Dart peer to drop
its reference to the SingleFrameCodec and potentially delete the
SingleFrameCodec.

Fixes https://github.com/flutter/flutter/issues/36631

Change ParagraphBuilder to replace the parent style's font families with the child style's font families (#10074)

Previously ParagraphBuilder was concatenating the child's fonts to the
parent's font list, causing the parent's fonts to take precedence.

Fixes https://github.com/flutter/flutter/issues/35992

After the frame is decoded SingleFrameCodec will invoke pending callbacks
from calls to getNextFrame.  The callbacks could result in more calls to
getNextFrame.  Those calls should receive the frame instead of being added
to the pending list.

Fixes https://github.com/flutter/flutter/issues/36312

Adds a unit-test asserting this behavior.

The codec will retain a reference to the decoded frame so it can be returned
in repeated calls to getNextFrame.

Also change the compressed image buffer to use an anonymous mapping on Android.
This is a workaround for high consumption of native heap pages by Android's
malloc when buffers are allocated on one thread and freed on another thread.

See https://github.com/flutter/flutter/issues/36079

Made Picture::toImage happen on the IO thread with no need for a surface.

Add typography sample from the Flutter 1.7 announcement blog. 

Fixes UB which caused failures on Windows on the unit-test run.

This converts the libtxt Paragraph and ParagraphBuilder classes into
interfaces with Minikin and SkShaper/SkParagraph based implementations.

Use the --enable-skshaper GN flag to select the Skia shaper implementation
at build time.

* Roll src/third_party/dart 06c3d7ad3a...09fc76bc51

dart-lang/sdk@09fc76bc51 [vm, compiler] Remove dead _classRangeCheckNegative.
dart-lang/sdk@b472d7a9be Unnecessary null aware spread hint
dart-lang/sdk@f939ad3964 library scope extensions
dart-lang/sdk@9503969664 [vm/bytecode] Add DebugCheck bytecode instruction
dart-lang/sdk@fc542be6b4 Issue 36682. Check that selection offset/length is valid in Extract Method refactoring.
dart-lang/sdk@803658a6d8 [corelib_2] fix bigint_from_test for web platforms
dart-lang/sdk@ad6b1ebbd6 Support for 'double' fields in protocol.
dart-lang/sdk@5479175012 Remove incorrect test.
dart-lang/sdk@058092b5f6 [vm] Fix simarm_x64 build on mac
dart-lang/sdk@307ca3f3b6 [vm/bytecode] Improve single stepping and breakpoint setting in bytecode.
dart-lang/sdk@855830f324 Update LSP spec
dart-lang/sdk@d992f55094 [dartfix] Bump pedantic dep to v1.8.0 and cleanup lint violations
dart-lang/sdk@41330f3e34 gitignore .clangd
dart-lang/sdk@8d07009931 [ VM / Gardening ] Mark disassemble_determinism_test as slow on Windows
dart-lang/sdk@1b82367ed6 Update CHANGELOG to reflect breaking change #36765
dart-lang/sdk@a86db84e0a [fasta] Preserve information about const constructors in outline.
dart-lang/sdk@25319ef7a6 Fix bug in codegen/string_escapes_test.dart.
dart-lang/sdk@7acecda2cc [vm/ffi] Fix FFI Utf8 example.
dart-lang/sdk@e3b3c6fa28 [vm] Rename 3head flutter patch after re-land of concurrency change
dart-lang/sdk@2cb5303782 [infra] Update checked in SDKs to 2.4.0
dart-lang/sdk@fce43ebc4e [ddk] Pass environmentDefines to CFE through CompilerOptions
dart-lang/sdk@5450d08ca1 Add support for analyzer static error tests.
dart-lang/sdk@0425997b31 Second attempt to reland "[vm/concurrency] Introduce concept of Isolate Groups"
dart-lang/sdk@5470159054 [vm, gc] Produce a proper error message when crashing due to lack of memory at isolate startup.
dart-lang/sdk@bfc7d21da6 Remove useless dart2js_extra/class_test
dart-lang/sdk@572619b639 [vm, bytecode] Fix vm/cc/CompileFunctionOnHelperThread.
dart-lang/sdk@4de495fccc [vm, compiler] Remove dead stub ICCallThroughFunction.
dart-lang/sdk@8a1dcdae68 Graduate 'flutter' domain from experimental, remove unused methods and fields.
dart-lang/sdk@a945888627 Discard constructors and fields temporarily to get valid code compiling

* Fix analyzer warning.

This patch reworks image decompression and collection in the following ways
because of misbehavior in the described edge cases.

The current flow for realizing a texture on the GPU from a blob of compressed
bytes is to first pass it to the IO thread for image decompression and then
upload to the GPU. The handle to the texture on the GPU is then passed back to
the UI thread so that it can be included in subsequent layer trees for
rendering. The GPU contexts on the Render & IO threads are in the same
sharegroup so the texture ends up being visible to the Render Thread context
during rendering. This works fine and does not block the UI thread. All
references to the image are owned on UI thread by Dart objects. When the final
reference to the image is dropped, the texture cannot be collected on the UI
thread (because it has not GPU context). Instead, it must be passed to either
the GPU or IO threads. The GPU thread is usually in the middle of a frame
workload so we redirect the same to the IO thread for eventual collection. While
texture collections are usually (comparatively) fast, texture decompression and
upload are slow (order of magnitude of frame intervals).

For application that end up creating (by not necessarily using) numerous large
textures in straight-line execution, it could be the case that texture
collection tasks are pending on the IO task runner after all the image
decompressions (and upload) are done. Put simply, the collection of the first
image could be waiting for the decompression and upload of the last image in the
queue.

This is exacerbated by two other hacks added to workaround unrelated issues.
* First, creating a codec with a single image frame immediately kicks of
  decompression and upload of that frame image (even if the frame was never
  request from the codec). This hack was added because we wanted to get rid of
  the compressed image allocation ASAP. The expectation was codecs would only be
  created with the sole purpose of getting the decompressed image bytes.
  However, for applications that only create codecs to get image sizes (but
  never actually decompress the same), we would end up replacing the compressed
  image allocation with a larger allocation (device resident no less) for no
  obvious use. This issue is particularly insidious when you consider that the
  codec is usually asked for the native image size first before the frame is
  requested at a smaller size (usually using a new codec with same data but new
  targetsize). This would cause the creation of a whole extra texture (at 1:1)
  when the caller was trying to “optimize” for memory use by requesting a
  texture of a smaller size.
* Second, all image collections we delayed in by the unref queue by 250ms
  because of observations that the calling thread (the UI thread) was being
  descheduled unnecessarily when a task with a timeout of zero was posted from
  the same (recall that a task has to be posted to the IO thread for the
  collection of that texture). 250ms is multiple frame intervals worth of
  potentially unnecessary textures.

The net result of these issues is that we may end up creating textures when all
that the application needs is to ask it’s codec for details about the same (but
not necessarily access its bytes). Texture collection could also be delayed
behind other jobs to decompress the textures on the IO thread. Also, all texture
collections are delayed for an arbitrary amount of time.

These issues cause applications to be susceptible to OOM situations. These
situations manifest in various ways. Host memory exhaustion causes the usual OOM
issues. Device memory exhaustion seems to manifest in different ways on iOS and
Android. On Android, allocation of a new texture seems to be causing an
assertion (in the driver). On iOS, the call hangs (presumably waiting for
another thread to release textures which we won’t do because those tasks are
blocked behind the current task completing).

To address peak memory usage, the following changes have been made:
* Image decompression and upload/collection no longer happen on the same thread.
  All image decompression will now be handled on a workqueue. The number of
  worker threads in this workqueue is equal to the number of processors on the
  device. These threads have a lower priority that either the UI or Render
  threads. These workers are shared between all Flutter applications in the
  process.
* Both the images and their codec now report the correct allocation size to Dart
  for GC purposes. The Dart VM uses this to pick objects for collection. Earlier
  the image allocation was assumed to 32bpp with no mipmapping overhead
  reported. Now, the correct image size is reported and the mipmapping overhead
  is accounted for. Image codec sizes were not reported to the VM earlier and
  now are. Expect “External” VM allocations to be higher than previously
  reported and the numbers in Observatory to line up more closely with actual
  memory usage (device and host).
* Decoding images to a specific size used to decode to 1:1 before performing a
  resize to the correct dimensions before texture upload. This has now been
  reworked so that images are first decompressed to a smaller size supported
  natively by the codec before final resizing to the requested target size. The
  intermediate copy is now smaller and more promptly collected. Resizing also
  happens on the workqueue worker.
* The drain interval of the unref queue is now sub-frame-interval. I am hesitant
  to remove the delay entirely because I have not been able to instrument the
  performance overhead of the same. That is next on my list. But now, multiple
  frame intervals worth of textures no longer stick around.

The following issues have been addressed:
* https://github.com/flutter/flutter/issues/34070 Since this was the first usage
  of the concurrent message loops, the number of idle wakes were determined to
  be too high and this component has been rewritten to be simpler and not use
  the existing task runner and MessageLoopImpl interface.
* Image decoding had no tests. The new `ui_unittests` harness has been added
  that sets up a GPU test harness on the host using SwiftShader. Tests have been
  added for image decompression, upload and resizing.
* The device memory exhaustion in this benchmark has been addressed. That
  benchmark is still not viable for inclusion in any harness however because it
  creates 9 million codecs in straight-line execution. Because these codecs are
  destroyed in the microtask callbacks, these are referenced till those
  callbacks are executed. So now, instead of device memory exhaustion, this will
  lead to (slower) exhaustion of host memory. This is expected and working as
  intended.

This patch only addresses peak memory use and makes collection of unused images
and textures more prompt. It does NOT address memory use by images referenced
strongly by the application or framework.

TextStyle.height property as a multiple of font size instead of multiple of ascent+descent+leading. (#9041)