* 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)
```
* Roll engine to c88ba37c794f57982374bb53a694301198cad593.
Changes since last roll:
- move generated entry points JSON out of flutter_patched_sdk
- List missing core lib bigint_patch.dart source for new Bigint implementation (#4735)
- Skip over .emf-files. (#4739)
- Roll dart to ee15c8eb689791f6777eb3f6b0a1e9c58ff0671b.
* defaultValue -> orElse
* Revert "Disable a text field test that fails on some Macs with libtxt (#14895)"
This reverts commit aa04a056f3.
* Revert "Roll engine to 33b88173f3820690169348859bbdc29133179e0b (#14832)"
This reverts commit 44592238bb.
* Revert non-engine reverts
Inlcudes:
* libtxt: specifically request the Roboto font in tests and benchmarks (flutter/engine#4675)
* Make Paragraph methods really return List<int> if they are declared as such (flutter/engine#4676)
* Stop using ClassHierarchy.deprecated_incremental (flutter/engine#4678)
* On iOS, render to an offscreen surface to avoid renderbuffer slowdown (flutter/engine#4680)
* Revert "Roll engine to 834d6eb02988e7f2c44eda05cec908378a1a9138 (#14664)"
This reverts commit c19fbe86dd.
Reason: timeouts while doing hot reload in hot_mode_dev_cycle__preview_dart_2_benchmark.
It seems that setTraversalBefore doesn't work as well as setTraversalAfter for some reason, although I'm using them the same way. Some apps would lock up TalkBack when traversing if setTraversalBefore was set, but not with the equivalent setTraversalAfter.
It's not entirely clear why this is, but I'm going with this to at least get it fixed for apps we know about.
Fixes#14600
Includes:
* Roll topaz to 531e2778d86b213bc663ea51bb0e0dde33d8ac8c (#4657)
This supports SDK/project paths including URI-escaped UTF-8 sequences
(e.g., including spaces, accented characters, etc.).
Includes:
* Roll topaz to 08a3394395036a2bb9b556f5b0eb8f365d2c0fa5 (flutter/engine#4650)
* [fuchsia] Add missing AOT entry points for FromFileResult and MapResult. (flutter/engine#4649)
* [fuchsia] Fix compile-time error introduced in 58e6c23. (flutter/engine#4651)
* Roll topaz to 046105efd225ed61c94e65dbd4a528256a3a34a9 (flutter/engine#4652)
Topaz rolls enable:
* Support for Flutter SDK, project paths including spaces (flutter/engine#4650)
* Support for Flutter SDK, project paths including arbitrary UTF-8 sequences (flutter/engine#4652)
* Roll engine to pre-dart roll
* Roll engine to pick up updated dart
* Apply Map changes
* Move to dev.22
* Fix some analysis issues
* Silent analyzer
* More consts
* More const massaging
* Yet more const massaging
* Yet more const massaging
* Use nonconst()
Includes:
* Only implement application:performActionForShortcutItem:completionHandler on iOS 9.0+ (flutter/engine#4600)
* [fuchsia] Remove MessageLoopHandler dependency (flutter/engine#4618)
* Use @available checks for iOS 11 features (#4619)
