Roll Dart SDK from e24733ebd16c to 243ad5427564 (5 revisions) (flutter#19338)

https://dart.googlesource.com/sdk.git/+log/e24733ebd16c..243ad5427564

2020-06-26 [email protected] analyzer_cli: Depend on pedantic directly
2020-06-26 [email protected] Clean up uses of @virtual in tests
2020-06-26 [email protected] Consolidate to a single implementation of refineBinaryExpressionType.
2020-06-26 [email protected] Migrate standalone_2 to null safety.
2020-06-26 [email protected] add fixnum deps patch for flutter hhh bots

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/dart-sdk-flutter-engine
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

Author: skia-flutter-autoroll

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': 'e24733ebd16c2cf3a839f3c51458c7f224cda487',
+  'dart_revision': '243ad54275648db6162d3932fab5960a6915fa0f',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py