Roll src/third_party/dart 3414b51..68e904e (17 commits) (#16367)

https://dart.googlesource.com/sdk.git/+log/3414b5167554..68e904e444dc

git log 3414b51..68e904e --date=short --first-parent --format='%ad %ae %s'
2020-02-04 rnystrom@google.com Migrate language_2/abstract to NNBD.
2020-02-04 jcollins@google.com Add a 'run' feature to MultiFutureTracker.
2020-02-04 kustermann@google.com [infa] Respect --sanitizer flag in tools/test.py when running GN
2020-02-04 rmacnak@google.com [vm] Speed-up the stack overflow check as used by reload stress testing.
2020-02-04 scheglov@google.com Issue 40392. Report SWITCH_CASE_COMPLETES_NORMALLY when switch/case completes normally.
2020-02-04 sstrickl@google.com Revert "[vm/aot] Remove object wrapping of bare instructions for AOT snapshots."
2020-02-04 sstrickl@google.com Revert "[gardening] Fix assert failure on (sim)arm_x64 due to 7475c63."
2020-02-04 scheglov@google.com Extract test for CASE_EXPRESSION_TYPE_IMPLEMENTS_EQUALS.
2020-02-04 asiva@google.com [VM/library] Fix signature of waitFor to account for potential null return value. This should fix the breakage in the NNBD on test standalone/io/issue_32052_test
2020-02-04 liama@google.com [test] Run late field tests in optimised mode
2020-02-04 sstrickl@google.com [gardening] Fix assert failure on (sim)arm_x64 due to 7475c63.
2020-02-04 rnystrom@google.com Migrate language_2/argument to NNBD.
2020-02-04 jensj@google.com [CFE] Parser test for class called late and required
2020-02-04 dmitryas@google.com [cfe] Disallow default List constructor invocations in NNBD
2020-02-04 johnniwinther@google.com [cfe] Update tests to show that inc/dec shorting is supported
2020-02-04 victor.agnez@hotmail.com [vm] Allow unboxed integer fields in AOT
2020-02-04 dmitryas@google.com [cfe] Disallow late final fields and const constructors

Created with:
  gclient setdep -r src/third_party/dart@68e904e444dc

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': '3414b516755486eea788117125e17d4f7c4e0861',
+  'dart_revision': '68e904e444dcfdce6acbce49e949df675f44674b',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py

ci/licenses_golden/licenses_third_party

@@ -1,4 +1,4 @@
-Signature: a4ee20f8eb7fb9c7ce2ac5d9348208b8
+Signature: d5e9bd72c17a7fa0003170ac337e5223
 
 UNUSED LICENSES: