CFE crashes on Enhanced Enums co19 tests

[sgrekhov]

CFE crashes on the following co 19 tests
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Enhanced-Enum/grammar_A10_t02.dart
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Enhanced-Enum/grammar_A10_t03.dart
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t02.dart
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Enhanced-Enum/implementing_enum_A03_t03.dart

/===============================================================================================================\
| cfe-strong-linux:co19/LanguageFeatures/Enhanced-Enum/grammar_A10_t02 is new and failed (Crash, expected Pass) |
\===============================================================================================================/

--- Command "fasta" (took 101ms):
(cd /b/s/w/ir/ ; DART_CONFIGURATION=ReleaseX64 ./out/ReleaseX64/dart ./pkg/front_end/tool/_fasta/compile.dart --verify --skip-platform-verification -o ./out/ReleaseX64/generated_compilations/cfe-strong-linux/tests_co19_src_LanguageFeatures_Enhanced-Enum_grammar_A10_t02/out.dill --platform ./out/ReleaseX64/vm_platform_strong.dill -Dtest_runner.configuration=cfe-strong-linux --enable-experiment=enhanced-enums --nnbd-strong --packages=./.packages ./tests/co19/src/LanguageFeatures/Enhanced-Enum/grammar_A10_t02.dart )

exit code:
253

stderr:
Unhandled exception:
  Crash when compiling null,
at character offset null:
type 'SourceProcedureBuilder' is not a subtype of type 'SourceFieldBuilder' in type cast
#0      SourceEnumBuilder.build (package:front_end/src/fasta/source/source_enum_builder.dart:500:36)
#1      SourceLibraryBuilder.buildBuilder (package:front_end/src/fasta/source/source_library_builder.dart:3013:31)
#2      SourceLibraryBuilder.build (package:front_end/src/fasta/source/source_library_builder.dart:1063:7)
#3      SourceLoader.buildComponent (package:front_end/src/fasta/source/source_loader.dart:1951:32)
#4      KernelTarget.buildOutlines.<anonymous closure> (package:front_end/src/fasta/kernel/kernel_target.dart:478:14)
<asynchronous suspension>
#5      withCrashReporting (package:front_end/src/fasta/crash.dart:122:12)
<asynchronous suspension>
#6      CompileTask._buildOutline (file:///b/s/w/ir/pkg/front_end/tool/_fasta/entry_points.dart:407:31)
<asynchronous suspension>
#7      CompileTask.compile (file:///b/s/w/ir/pkg/front_end/tool/_fasta/entry_points.dart:446:31)
<asynchronous suspension>
#8      BatchCompiler.batchCompileImpl (file:///b/s/w/ir/pkg/front_end/tool/_fasta/entry_points.dart:235:5)
<asynchronous suspension>
#9      runProtectedFromAbort (file:///b/s/w/ir/pkg/front_end/tool/_fasta/command_line.dart:400:12)
<asynchronous suspension>
#10     BatchCompiler.run (file:///b/s/w/ir/pkg/front_end/tool/_fasta/entry_points.dart:187:13)
<asynchronous suspension>


#0      SourceEnumBuilder.build (package:front_end/src/fasta/source/source_enum_builder.dart:500:36)
#1      SourceLibraryBuilder.buildBuilder (package:front_end/src/fasta/source/source_library_builder.dart:3013:31)
#2      SourceLibraryBuilder.build (package:front_end/src/fasta/source/source_library_builder.dart:1063:7)
#3      SourceLoader.buildComponent (package:front_end/src/fasta/source/source_loader.dart:1951:32)
#4      KernelTarget.buildOutlines.<anonymous closure> (package:front_end/src/fasta/kernel/kernel_target.dart:478:14)
<asynchronous suspension>
#5      withCrashReporting (package:front_end/src/fasta/crash.dart:122:12)
<asynchronous suspension>
#6      CompileTask._buildOutline (file:///b/s/w/ir/pkg/front_end/tool/_fasta/entry_points.dart:407:31)
<asynchronous suspension>
#7      CompileTask.compile (file:///b/s/w/ir/pkg/front_end/tool/_fasta/entry_points.dart:446:31)
<asynchronous suspension>
#8      BatchCompiler.batchCompileImpl (file:///b/s/w/ir/pkg/front_end/tool/_fasta/entry_points.dart:235:5)
<asynchronous suspension>
#9      runProtectedFromAbort (file:///b/s/w/ir/pkg/front_end/tool/_fasta/command_line.dart:400:12)
<asynchronous suspension>
#10     BatchCompiler.run (file:///b/s/w/ir/pkg/front_end/tool/_fasta/entry_points.dart:187:13)
<asynchronous suspension>

--- Re-run this test:
python3 tools/test.py -n cfe-strong-linux co19/LanguageFeatures/Enhanced-Enum/grammar_A10_t02

Tested on Dart SDK version: 2.17.0-edge.4fb3d2afcf424ff3f73d55fc1585c448cdaa8762 (be) (Thu Feb 10 03:16:20 2022 +0000) on "linux_x64"