Roll Dart SDK from 3d53df52afb5 to 54adfeb93f0b (14 revisions) (#18747)

https://dart.googlesource.com/sdk.git/+log/3d53df52afb5..54adfeb93f0b

2020-06-02 devoncarew@google.com [nnbd_migration] handle multiple contexts
2020-06-02 zichangguo@google.com [dart:io] Fixes the FileIO performance regression
2020-06-02 srawlins@google.com Analyzer: move built_in_identifier_* tests
2020-06-02 rnystrom@google.com Migrate language_2/metadata to NNBD.
2020-06-02 rnystrom@google.com Migrate language_2/malformed to NNBD.
2020-06-02 srawlins@google.com Analyzer: Mark used extension operators as such
2020-06-02 srawlins@google.com Migrator: Edit the package_config.json file
2020-06-02 scheglov@google.com Clean up verify_sorted_test.
2020-06-02 paulberry@google.com Revert "Remove DartSdk.useSummary."
2020-06-02 jwren@google.com Cleanup format.dart now that recent dartfmt has been rolled forward
2020-06-02 danny@tuppeny.com Enable/disable and register/unregister LSP formatter based on client settings
2020-06-02 danny@tuppeny.com Add LSP support for client-side configuration
2020-06-02 danny@tuppeny.com Minor refactor of LSP server capabilities+init test
2020-06-02 danny@tuppeny.com Improve LSP spec regex to catch didChangeConfiguration method

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

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': '3d53df52afb56e797255125e3b038112ce6395be',
+  'dart_revision': '54adfeb93f0bd14a150b774ea80b799be6004137',
 
   # 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

-Signature: b319b4bf8359db6264e9ca471aaf2e74
+Signature: 988852c113dcca729815b892b258fa21
 
 UNUSED LICENSES: