[versions] update to latest source span and roll engine to 4b8477d11573d233e6791204191c0090f733b05d

[jonahwilliams]

Description

flutter/engine@2abe69c...4b8477d

2020-09-17 [email protected] Roll Dart SDK from fcaedc6d9587 to 2cec6af2652f (1 revision) (flutter/engine#21257)
2020-09-17 [email protected] Disable ShellTest.SkipAndSubmitFrame for flakes (flutter/engine#21254)
2020-09-17 [email protected] Roll Skia from 45f41b376260 to c21dc07a78b9 (4 revisions) (flutter/engine#21250)
2020-09-17 [email protected] Add --enable-isolate-groups to allowed flags in switches.cc (flutter/engine#21149)
2020-09-17 [email protected] [web] Remove commented import (flutter/engine#21247)
2020-09-17 [email protected] Roll Skia from 12d06a38427d to 45f41b376260 (6 revisions) (flutter/engine#21249)
2020-09-17 [email protected] Add support of cross-building the engine for ARM64 Linux Host (flutter/engine#20254)
2020-09-17 [email protected] Roll Fuchsia Linux SDK from 91U3isvKn... to Qi0ptKLxN... (flutter/engine#21246)
2020-09-17 [email protected] [web] run safari desktop tests on luci (flutter/engine#21228)
2020-09-17 [email protected] avoid hard coding OS (flutter/engine#21248)
2020-09-17 [email protected] Roll Dart SDK from 99970d646dc9 to fcaedc6d9587 (3 revisions) (flutter/engine#21245)
2020-09-17 [email protected] [fuchsia] set maxframesinflight to be configurable (flutter/engine#21152)
2020-09-17 [email protected] Roll Skia from 51a7f9559ad4 to 12d06a38427d (3 revisions) (flutter/engine#21244)
2020-09-17 [email protected] Roll Skia from f9fdf291c567 to 51a7f9559ad4 (5 revisions) (flutter/engine#21242)
2020-09-17 [email protected] Roll Dart SDK from eb24e3324908 to 99970d646dc9 (1 revision) (flutter/engine#21239)
2020-09-17 [email protected] Roll Fuchsia Mac SDK from 13BzEPO47... to 2nHpC_AEO... (flutter/engine#21238)
2020-09-17 [email protected] Roll Skia from bf66ffbbd4ce to f9fdf291c567 (1 revision) (flutter/engine#21235)
2020-09-17 [email protected] Roll Dart SDK from e41a6008746d to eb24e3324908 (1 revision) (flutter/engine#21233)
2020-09-17 [email protected] Roll Fuchsia Linux SDK from G5UItrFeP... to 91U3isvKn... (flutter/engine#21232)
2020-09-17 [email protected] Roll Skia from 2564767d24e5 to bf66ffbbd4ce (27 revisions) (flutter/engine#21230)

Also rolling transitive DEPS:
fuchsia/sdk/core/linux-amd64 from G5UItrFeP96D to Qi0ptKLxNLDk
fuchsia/sdk/core/mac-amd64 from 13BzEPO47Hu7 to 2nHpC_AEOz0o

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/flutter-engine-flutter-autoroll
Please CC [email protected] on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/master/autoroll/README.md

Fixes #66132

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[zanderso]

This lgtm. Please wait to land until the tree is greener.

[jonahwilliams]

FYI path_provider windows broke web compilation. it looks like it dart:ffi is reachable where it wasn't before. @stuartmorgan

[web_benchmarks_html] [STDOUT] Executing: /b/s/w/ir/k/flutter/bin/flutter build web --dart-define=FLUTTER_WEB_ENABLE_PROFILING=true --profile -t lib/web_benchmarks.dart in /b/s/w/ir/k/flutter/dev/benchmarks/macrobenchmarks
[web_benchmarks_html] [STDOUT] stdout: Downloading Web SDK...                                              1.0s
[web_benchmarks_html] [STDOUT] stdout: Compiling lib/web_benchmarks.dart for the Web...                
[web_benchmarks_html] [STDOUT] stderr: Target dart2js failed: Exception: ../../../../../cache/.pub-cache/hosted/pub.dartlang.org/path_provider_windows-0.0.2/lib/path_provider_windows.dart:7:8:
[web_benchmarks_html] [STDOUT] stderr: Error: Not found: 'dart:ffi'
[web_benchmarks_html] [STDOUT] stderr: import 'dart:ffi';
[web_benchmarks_html] [STDOUT] stderr:        ^
[web_benchmarks_html] [STDOUT] stderr: ../../../../../cache/.pub-cache/hosted/pub.dartlang.org/ffi-0.1.3/lib/src/utf8.dart:6:8:
[web_benchmarks_html] [STDOUT] stderr: Error: Not found: 'dart:ffi'
[web_benchmarks_html] [STDOUT] stderr: import 'dart:ffi';
[web_benchmarks_html] [STDOUT] stderr:        ^
[web_benchmarks_html] [STDOUT] stderr: ../../../../../cache/.pub-cache/hosted/pub.dartlang.org/ffi-0.1.3/lib/src/utf16.dart:6:8:
[web_benchmarks_html] [STDOUT] stderr: Error: Not found: 'dart:ffi'
[web_benchmarks_html] [STDOUT] stderr: import 'dart:ffi';
[web_benchmarks_html] [STDOUT] stderr:        ^
[web_benchmarks_html] [STDOUT] stderr: ../../../../../cache/.pub-cache/hosted/pub.dartlang.org/ffi-0.1.3/lib/src/allocation.dart:5:8:
[web_benchmarks_html] [STDOUT] stderr: Error: Not found: 'dart:ffi'
[web_benchmarks_html] [STDOUT] stderr: import 'dart:ffi';
[web_benchmarks_html] [STDOUT] stderr:        ^
[web_benchmarks_html] [STDOUT] stderr: ../../../../../cache/.pub-cache/hosted/pub.dartlang.org/win32-1.7.1/lib/src/callbacks.dart:7:8: