Revert "Turn support for sync-async on in Flutter. (#16543)" (#16626)

This reverts commit b0b2c0077a56e313cb00a9c5285e4bf4fb43f3b0.

dev/automated_tests/flutter_test/exception_handling_expectation.txt

diff --git a/dev/automated_tests/flutter_test/exception_handling_expectation.txt b/dev/automated_tests/flutter_test/exception_handling_expectation.txt
index 9e1affd..4c0fb6e 100644
--- a/dev/automated_tests/flutter_test/exception_handling_expectation.txt
+++ b/dev/automated_tests/flutter_test/exception_handling_expectation.txt
@@ -43,7 +43,7 @@
 
 When the exception was thrown, this was the stack:
 #[0-9]+ +main.<anonymous closure> \(.+[/\\]dev[/\\]automated_tests[/\\]flutter_test[/\\]exception_handling_test\.dart:16:9\)
-<<skip until matching line>>
+#[0-9]+ +main.<anonymous closure> \(.+[/\\]dev[/\\]automated_tests[/\\]flutter_test[/\\]exception_handling_test\.dart:15:77\)
 #[0-9]+ +.+ \(package:flutter_test[/\\]src[/\\]widget_tester\.dart:[0-9]+:[0-9]+\)
 <<skip until matching line>>
 ^\(elided [0-9]+ .+\)$

dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt

diff --git a/dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt b/dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt
index 6c8afe8..6cb6a93 100644
--- a/dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt
+++ b/dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt
@@ -18,10 +18,6 @@
 The test description was:
 TestAsyncUtils - custom guarded sections
 ════════════════════════════════════════════════════════════════════════════════════════════════════
-══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
-The following message was thrown:
-Multiple exceptions \(2\) were detected during the running of the current test, and at least one was unexpected.
-════════════════════════════════════════════════════════════════════════════════════════════════════
 .*(this line has more of the test framework's output)?
   Test failed\. See exception logs above\.
   The test description was: TestAsyncUtils - custom guarded sections

packages/flutter_tools/lib/src/compile.dart

diff --git a/packages/flutter_tools/lib/src/compile.dart b/packages/flutter_tools/lib/src/compile.dart
index 658cf78..4e3d3ba 100644
--- a/packages/flutter_tools/lib/src/compile.dart
+++ b/packages/flutter_tools/lib/src/compile.dart
@@ -90,7 +90,6 @@
     '--sdk-root',
     sdkRoot,
     '--strong',
-    '--sync-async',
     '--target=flutter',
   ];
   if (trackWidgetCreation)
@@ -216,7 +215,6 @@
       _sdkRoot,
       '--incremental',
       '--strong',
-      '--sync-async',
       '--target=flutter',
     ];
     if (outputPath != null) {