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

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.kernel_enum_builder;
 
 import 'package:kernel/ast.dart'
     show
         Arguments,
         Class,
@@ skipping 222 matching lines @@
 
   KernelTypeBuilder get mixedInType => null;
 
   InterfaceType buildType(
       LibraryBuilder library, List<KernelTypeBuilder> arguments) {
     return cls.rawType;
   }
 
   @override
   Class build(KernelLibraryBuilder libraryBuilder, LibraryBuilder coreLibrary) {
+    cls.isEnum = true;
     if (constantNamesAndOffsets.isEmpty) {
       libraryBuilder.addCompileTimeError(
           messageEnumDeclartionEmpty, charOffset, fileUri);
     }
     intType.resolveIn(coreLibrary.scope);
     stringType.resolveIn(coreLibrary.scope);
     objectType.resolveIn(coreLibrary.scope);
     listType.resolveIn(coreLibrary.scope);
     toStringMap.keyType = intType.build(libraryBuilder);
     toStringMap.valueType = stringType.build(libraryBuilder);
@@ skipping 53 matching lines @@
     }
     return super.build(libraryBuilder, coreLibrary);
   }
 
   @override
   Builder findConstructorOrFactory(
       String name, int charOffset, Uri uri, LibraryBuilder library) {
     return null;
   }
 }