Add Class.isEnum to Kernel and use it to resynthesize enums in Analyzer.

pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
index fadda95d116b6ad8cc5a57b8fb896842d5aeccd4..fe3f792e5b45f1396e17cc959169078c08180e4c 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
@@ -240,6 +240,7 @@ class KernelEnumBuilder extends SourceClassBuilder
 
   @override
   Class build(KernelLibraryBuilder libraryBuilder, LibraryBuilder coreLibrary) {
+    cls.isEnum = true;
     if (constantNamesAndOffsets.isEmpty) {
       libraryBuilder.addCompileTimeError(
           messageEnumDeclartionEmpty, charOffset, fileUri);