Roll src/third_party/dart aa2b3a5..521b168 (15 commits) (flutter#13958)

dart-lang/sdk@521b168441 [co19] Add tests/co19_2/co19_2-co19.status to test_matrix.json
dart-lang/sdk@8295efc541 Revert "Revamp patch_sdk.dart mainly to use libraries.json."
dart-lang/sdk@acff31ed26 [cfe] Support null-aware index access
dart-lang/sdk@f8d42542dd [vm/compiler] ARM64: Block R22 to hold NullObject().
dart-lang/sdk@afa82d8e98 [co19] Add missing backslash to update script
dart-lang/sdk@28a029102b [SDK] Adds --support-disassembler in PRODUCT.
dart-lang/sdk@45569e146e [SDK] Fixes C++ undef. behaviour in Library::Add*Metadata.
dart-lang/sdk@51f14eb911 [co19] Fork the update script for the co19 NNBD tests
dart-lang/sdk@eb6d7f51e2 [dart2js] Add DartTypes for T*, T?, and Never.
dart-lang/sdk@4c8c44ec78 [cfe] Implement lowering for constructor initialization of late fields
dart-lang/sdk@f692a2fbb8 [cfe] Eliminate Nullability.legacy in transformers
dart-lang/sdk@539b0093e1 [dart2js] Remove unnecessary DartType predicates and replace with is-tests.
dart-lang/sdk@2e32972929 [pkg/vm] Remove usage of Library.isExternal
dart-lang/sdk@54011e5622 [analyzer] Fix format.fbs after summary change for variance.
dart-lang/sdk@615928c514 Stop referencing non-API TypeSystem in analyzer.

Author: bkonyi

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': 'aa2b3a5eaa4adb0d11eabdcc15c4056a3dc8602a',
+  'dart_revision': '521b168441143861d928c9315c9830469617028b',
 
   # 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: 7da493d646e9f99c5b1b71762f1fe64d
+Signature: f7334fbbdc2435fe4814f8135afa73d6
 
 UNUSED LICENSES:
 
@@ -8059,6 +8059,7 @@ FILE: ../../../third_party/dart/samples/sample_extension/test_sample_synchronous
 FILE: ../../../third_party/dart/samples/samples.status
 FILE: ../../../third_party/dart/sdk/lib/_http/crypto.dart
 FILE: ../../../third_party/dart/sdk/lib/_http/http_date.dart
+FILE: ../../../third_party/dart/sdk/lib/_internal/js_dev_runtime/libraries.dart
 FILE: ../../../third_party/dart/sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart
 FILE: ../../../third_party/dart/sdk/lib/_internal/js_dev_runtime/patch/core_patch.dart
 FILE: ../../../third_party/dart/sdk/lib/_internal/js_dev_runtime/patch/isolate_patch.dart
@@ -8184,6 +8185,7 @@ FILE: ../../../third_party/dart/sdk/lib/web_gl/dart2js/web_gl_dart2js.dart
 FILE: ../../../third_party/dart/sdk/lib/web_sql/dart2js/web_sql_dart2js.dart
 FILE: ../../../third_party/dart/sdk_nnbd/lib/_http/crypto.dart
 FILE: ../../../third_party/dart/sdk_nnbd/lib/_http/http_date.dart
+FILE: ../../../third_party/dart/sdk_nnbd/lib/_internal/js_dev_runtime/libraries.dart
 FILE: ../../../third_party/dart/sdk_nnbd/lib/_internal/js_dev_runtime/patch/async_patch.dart
 FILE: ../../../third_party/dart/sdk_nnbd/lib/_internal/js_dev_runtime/patch/core_patch.dart
 FILE: ../../../third_party/dart/sdk_nnbd/lib/_internal/js_dev_runtime/patch/isolate_patch.dart