[dart] Roll dart to a50dfd66980ed5c9aef9851fcc57557fb607769c.

Changes since last roll:

```
a50dfd6698 Make --write-results not appear in reproduction commands
6c0b854477 [VM] Fix canonicalization of doubles: Two doubles are canonicalized to the same value if they are idencial()
f4ebebb37b Ignore runtime arguments in Fasta compiler configuration
1f916eb4e6 Mark a failing test
296c99632d Remove outdated information from pkg/kernel/README.md
6cd25af684 Configure defines correctly in config_import_test.dart
4e4e9ec74a Implement int-to-double literal conversion.
79e8b3d57f Make importing two libraries with the same name a warning
89d753588c [test] Speed up get_vm_timeline_rpc_test.dart.
de18295924 Adjust some status files for the new build configurations added to the buildbots.
219dc1be87 Rework the class hierarchy of StrongModeStaticTypeAnalyzer2Test.
dd7fec41ec Issue 34693. Use isOverrideSubtypeOf() to check that the implemented member is a valid override of the member from the super-constraint.
26cab0a4db [VM/IO] The exit code handler thread should not try to get the exit codes for detached child processes. Currently we end up in a tight loop in the exit code handler when detached processes are created.
6edc759798 [vm] Do not print if kernel isolate initialization is interrupted by VM shutdown.
c8bda47d88 [standalone] Fix memory leaks when kernel isolate fails to launch from an app snapshot.
33485f8ef2 [standalone] Fix leak when the kernel service produces a compile-time error.
7f531b5112 Analyzer: Fix missing errors during (a)sync(*) yield of void.
4486bf5f6b [vm/bytecode] Fix return value for several BigInt intrinsics
28780d34df Convert element.dart to triple-slash comment style.
b4b02a6bde [vm] Include bytecode in core-jit and app-jit snapshots
2a3a5c88c7 [dart2js] Omit empty parts in --fast-startup
f939503b50 [vm/aot] Make mixin deduplication transformation more robust wrt new mixins
5ea1d2fb20 Convert idl.dart to triple-slash comment style.
69550e2837 [dart/fuzzer] Increase #tests to 125
5b1444503e [dart/fuzzer] Minor output tweaks
41a9c38433 [vm, compiler] Shrink deopt stub and runtime call sequences.
f96362ccfd [dart2js] Smaller code by caching values
b2d22be0f3 [vm, arm64] Take advantage of Fuchsia's lack of signal handlers.
ca8809fcfa Don't use variableElement.computeConstantValue() in ConstantVisitor.
9d150c6443 Avoid a downcast error
0566f5fa27 Remove unnecessary test status lines
0e7ca953a9 [gardening] Skip timing out bigint tests on slow configurations (#31659)
127c3fed6c Handle data URIs in dart2js
```