Revert to last-known-good-rev of Dart SDK (flutter#19031)

* Revert "Roll Dart SDK from 0b64f5488965 to 50836c171e91 (4 revisions) (flutter#19017)"

This reverts commit f6455fa.

* Revert "Roll Dart SDK from f0ea02bc51f8 to 0b64f5488965 (9 revisions) (flutter#19013)"

This reverts commit 1f3aa23.

* Revert "Roll Dart SDK from f043f9e5f6ea to f0ea02bc51f8 (22 revisions) (flutter#19010)"

This reverts commit 5f37029.

Author: George Wright

DEPS

@@ -34,7 +34,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '50836c171e9153459aafef4a2404b3180fb35fd6',
+  'dart_revision': 'f043f9e5f6ea4194c0ff4114e30adc5380cbc932',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py
@@ -354,7 +354,7 @@ deps = {
    Var('dart_git') + '/package_config.git@9c586d04bd26fef01215fd10e7ab96a3050cfa64',
 
   'src/third_party/dart/tools/sdks':
-   {'packages': [{'version': 'version:2.9.0-10.0.dev', 'package': 'dart/dart-sdk/${{platform}}'}], 'dep_type': 'cipd'},
+   {'packages': [{'version': 'version:2.9.0-15.0.dev', 'package': 'dart/dart-sdk/${{platform}}'}], 'dep_type': 'cipd'},
 
   # WARNING: end of dart dependencies list that is cleaned up automatically - see create_updated_flutter_deps.py.
 

ci/licenses_golden/licenses_third_party

@@ -1,4 +1,4 @@
-Signature: bfcb15c7a644429b76382d47bc5aa5d7
+Signature: e2381fb0c6bd8969b9e68bc7898a2915
 
 UNUSED LICENSES: