This engine roll contains the following commits:
* flutter/engine@09d05a389: Prepare and run child isolates on a VM managed (i.e. no UI) task runner.
* flutter/engine@adee35142: Throw an error or return null if FlutterView.getBitmap fails
This contains the following commits:
flutter/engine@3cc8d82 Fix more Linux unittest targets.
flutter/engine@b6ba69b libtxt: add boxes representing empty lines in GetRectsForRange
flutter/engine@388f019 Enable downscale of very large images when uploading on IO thread
flutter/engine@22e1381 Allow runtime unit-test to perform dynamic symbol resolution for Dart snapshots.
flutter/engine@59d1366 Don’t build the host shell on target builds.
flutter/engine@7a6b779 Remove the weak pointer factory from the service isolate's DartIsolate object
flutter/engine@d4729ab Update the embedder GN target to use a source set instead of a complete static library.
flutter/engine@8973c73 Implement Scene::toImage for creating a raster image representation of a scene.
flutter/engine@572c584 Configure kernel paths in the embedder if the asset directory contains the kernel assets
flutter/engine@8ed932c Roll src/third_party/skia/ 1cd6098d5..1ccaa6e05 (20 commits)
flutter/engine@a0804f3 Simplify weak pointer factories and allow vending weak pointers on multiple threads.
* Handle error count reported by frontend.
Extend compilation result from single string to a structure(string filename and integer error count).
* Use ?.
* Include engine roll with dart sdk roll.
* parse(onError) -> tryParse
* Make '?? throw' more readable and avoid issue with analyzer
* Fix test so it mocks compiler output including errors count
Changes since last roll:
```
ed303c628 Roll dart sdk again. Previous roll required 23ae4fa098 revert. (#4966)
8cd272733 Revert "Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)" (#4965)
9199b40f2 Revert "Support multiple shells in a single process. (#4932)" (#4964)
6baff4c82 Support multiple shells in a single process. (#4932)
31c5bb427 Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)
c8e4c6984 Avoid copying the contents of large platform message responses (#4947)
5ff527295 Update to use new vulkan GrBackendRenderTarget ctor. (#4962)
0c8993a1a Update to use new vulkan GrBackendRenderTarget ctor (part 2) (#4963)
132ebdda8 Revert "Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)" (#4961)
11882ab9e Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)
```
Add consts
Includes:
* Revert "Roll dart to 4bcf8ad411bd90da16d44ad399bf350e8423eac9" (flutter/engine#4936)
* libtxt: render fake bold text if a bold font is not available (flutter/engine#4933)
* Set FlutterTexture copyPixelBuffer return nullable (flutter/engine#4934)
* Roll dart to 4bcf8ad411bd90da16d44ad399bf350e8423eac9 (flutter/engine#4929)
Includes:
* libtxt: add missing dependency for Mac/iOS builds (flutter/engine#4931)
* libtxt: use Helvetica as a fallback font on iOS 8 and earlier (flutter/engine#4930)
* Make Rect.largest match the "largest" rect used in DefaultLayerBuilder (flutter/engine#4926)
* [vulkan] Add VK_ERROR_NOT_PERMITTED_EXT
* Roll buildroot to 8dddd90bf9 (flutter/engine#4928)
* Set the asset bundle path when initializing the shell in the embedder API (flutter/engine#4925)
Changes since last roll:
```
ec1e96864 Revert pub roll from recent dart roll. (#4904)
ac552fba4 Port flutter content handler to FIDL2. (#4903)
94cdd8103 Roll dart. (#4902)
af63732de Support `Path.addCircle` (#4783)
564c6a5a9 Add hint to use ninja with goma (#4894)
```
```
Includes:
* libtxt: move to the next run if the current run ends before the start of the line block (flutter/engine#4891)
* Rolled Dart SDK to e3051175192c83fafa3636c0f16d804734bb285a (flutter/engine#4877)
* roll engine version to 6280adbfb1f9f63cdc6179b9b78634add1e4f2e3
* Correctly handle previewDart : false setting in AnalysisServer and
AnalyzeOnce classes.
Includes:
* Provide lookup key to access Flutter assets in the APK (flutter/engine#4785)
* Roll skia and update licenses (flutter/engine#4797)
* libtxt: retain hard line breaks when calculating maxIntrinsicWidth (flutter/engine#4796)
* iupdate engine version to 141176b9004d7b01325629fd45b2ef61b0d00251
* update engine version to f192c59d76c0afe63f2290c3ed76061f00c77cc7
This rolls dart to version 0175f24b2549732d0010708988fd328440c3f956
* Move dart 2 tests back to optional section to allow debugging of a new
regression in the petitParser package.
* Update
* Roll engine to b6df7a637498ca9beda1fa9cd7210e3202ea599f.
Changes since last roll:
```
b6df7a637 Roll dart to 290c576264faa096a0b3206c71b2435309d9f904. (#4771)
a6764dbd5 Add sources for Fuchsia target. (#4763)
2d5900615 [fuchsia] Remove unused header file. (#4769)
9717063b7 Revert "Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)" (#4768)
9a9814312 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)
e74e8b35c [async] Update includes of async headers to new path (#4760)
e2c4b2760 Use Dart 2 camel case constants in the engine Dart libraries (#4766)
9c1e48434 Updates for Fuchsia roll. (#4765)
14c940e27 Switch from fxl::Mutex to std::mutex (#4764)
debf82c0b Roll Garnet (#4759)
5bffdefbb Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (#4761)
```
