Roll Dart SDK from f997d62a6d29 to 20bf2fcf5670 (18 revisions)

[skia-flutter-autoroll]

https://dart.googlesource.com/sdk.git/+log/f997d62a6d29..20bf2fcf5670

2020-07-14 kustermann@google.com [vm/compiler] Make precompiler prune invocation dispatchers the same way we do for regular/closure functions
2020-07-14 whesse@google.com [build] Change flutter-engine patch for async to use updated Dart SDK hash
2020-07-14 sortie@google.com Roll benchmarks-internal 78e4b5975ab4..9bfe209a53a5
2020-07-14 kustermann@google.com [vm] Remove unused function
2020-07-14 jakemac@google.com update async in the sdk, add flutter engine patch
2020-07-14 srawlins@google.com analyzer: Move tests for 9 codes to diagnostics/
2020-07-14 scheglov@google.com Prepare to publish analyzer 0.39.13
2020-07-14 regis@google.com [VM/nnbd] Class Future may be used for type normalization before it is cached in the object store.
2020-07-14 bkonyi@google.com [ package:dds ] Fixed expression evaluator expecting a double-nested 'result' from an external 'compileExpression' implementation
2020-07-14 nshahan@google.com [ddc] Copy more .dill files to packaged sdk
2020-07-13 nbosch@google.com Update to latest package:matcher
2020-07-13 aam@google.com [vm/bytecode] Ensure class is allocate-finalized before instantiated by bytecode runtime calls.
2020-07-13 kenzieschmoll@google.com Remove dart:io dep from utils.dart and add conditional imports for File
2020-07-13 bkonyi@google.com [ package:dds ] Fix issue where StateError could be raised when a client disconnects from DDS after the VM service has shutdown
2020-07-13 nshahan@google.com [ddc] Inline the sound null safety flag
2020-07-13 scheglov@google.com Support for data attachments to OperationPerformance.
2020-07-13 scheglov@google.com Instrument getAnalysisOptions() for Cider.
2020-07-13 scheglov@google.com Remove deprecated CiderOperationPerformance.

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

[a-siva]

Landing this CL as the flutter engine bot is turning green