Roll src/third_party/dart 6aa7407..e84bea2 (6 commits) (flutter#15934)

https://dart.googlesource.com/sdk.git/+log/6aa7407d6309..e84bea25df23

git log 6aa7407..e84bea2 --date=short --first-parent --format='%ad %ae %s'
2020-01-23 jensj@google.com [CFE] Fix crash in incremental compiler caused by async redirecting factory
2020-01-23 jensj@google.com [CFE] Incremental compiler refactor #1
2020-01-23 athom@google.com [release] Prepare CHANGELOG for 2.7.1
2020-01-23 sstrickl@google.com [vm] Refactor debug info handling code into a new package.
2020-01-23 jensj@google.com [CFE] Strong test can have linked dependencies
2020-01-23 johnniwinther@google.com [cfe] Check null nullable accesses and include null in inferred type for null aware expressions

Created with:
  gclient setdep -r src/third_party/dart@e84bea25df23

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 dart-vm-team@google.com 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/+/master/autoroll/README.md

Bug: None
Tbr: dart-vm-team@google.com

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': '6aa7407d63092696752c68dd0426c8ce99e5c6a8',
+  'dart_revision': 'e84bea25df23ea5f149fc808a6423c7500b9cace',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py