Roll Dart SDK from 63cf56d92510 to 648b5e9c6860 (19 revisions)

[skia-flutter-autoroll]

https://dart.googlesource.com/sdk.git/+log/63cf56d92510..648b5e9c6860

2020-06-26 srawlins@google.com Use ///-style doc comments in analyzer/lib
2020-06-26 jakemac@google.com update fixnum to nnbd version
2020-06-26 srawlins@google.com Use ///-style doc comments in analyzer/lib/src/generated
2020-06-26 cskau@google.com [Test] Skips sdk_hash_test for incompatible setups.
2020-06-26 srawlins@google.com Use ///-style doc comments in analyzer/lib/src/dart
2020-06-26 joshualitt@google.com [dart2js] 20 dart2js tests ported to nnbd #1.
2020-06-26 joshualitt@google.com [dart2js] Support package json files in modular_test_suite.
2020-06-26 cskau@google.com [Test] Fixes sdk_hash_test broken by Windows line breaks.
2020-06-26 cskau@google.com [SDK] Adds an SDK hash to kernels and the VM.
2020-06-26 johnniwinther@google.com [cfe] Update CFE message to use 'null safety' instead of 'non-nullable'
2020-06-26 dacoharkes@google.com [vm/ffi] Remove try-catch from ffi trampoline if no handle scope
2020-06-26 sstrickl@google.com Add internal benchmarks to .gitignore.
2020-06-26 karlklose@google.com [infra] Remove support for approvals from compare_results
2020-06-26 whesse@google.com Roll benchmarks-internal to 15a6634bd60ed0b937ed5dbc874cba2da9865514
2020-06-26 kustermann@google.com [vm] Remove ARMv6 related code
2020-06-26 sgrekhov@unipro.ru [co19] Enable Language/Libraries_and_Scripts tests
2020-06-26 athom@google.com [co19] Roll co19 to e674d8ccb29250fa5d7ac886fa98c5962543fffc
2020-06-26 jensj@google.com Revert "[CFE] Set correct async markers in outline phase"
2020-06-26 karlklose@google.com [infra] Support posting results to staging in post_results_to_pubsub

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/+doc/master/autoroll/README.md

[skia-flutter-autoroll]

AutoRoller is stopped; closing the active roll.