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

pkg/analyzer/lib/src/kernel/resynthesize.dart

 // Copyright (c) 2017, 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.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_ast_factory.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
@@ skipping 619 matching lines @@
         CompilationUnitElementImpl unitImpl = unit;
         ElementImpl child = unitImpl.getChild(name.name);
         if (child != null) {
           return child;
         }
       }
       return null;
     }
 
     // Search in the class.
-    if (parentElement is ClassElementImpl) {
+    if (parentElement is AbstractClassElementImpl) {
       if (isGetter) {
         return parentElement.getGetter(name.name) as ElementImpl;
       } else if (isSetter) {
         return parentElement.getSetter(name.name) as ElementImpl;
       } else if (isField) {
         return parentElement.getField(name.name) as ElementImpl;
       } else if (isConstructor) {
         if (name.name.isEmpty) {
           return parentElement.unnamedConstructor as ConstructorElementImpl;
         }
         return parentElement.getNamedConstructor(name.name) as ElementImpl;
       } else if (isMethod) {
         return parentElement.getMethod(name.name) as ElementImpl;
       }
       return null;
     }
 
     throw new UnimplementedError('Should not be reached.');
   }
 
   InterfaceType _getInterfaceType(ElementImpl context,
       kernel.CanonicalName className, List<kernel.DartType> kernelArguments) {
     var libraryName = className.parent;
     var libraryElement = _resynthesizer.getLibrary(libraryName.name);
-    ClassElementImpl classElement = libraryElement.getType(className.name);
+    ClassElement classElement = libraryElement.getType(className.name);
+    classElement ??= libraryElement.getEnum(className.name);
 
     if (kernelArguments.isEmpty) {
       return classElement.type;
     }
 
     return new InterfaceTypeImpl.elementWithNameAndArgs(
         classElement, classElement.name, () {
       List<DartType> arguments = kernelArguments
           .map((kernel.DartType k) => getType(context, k))
           .toList(growable: false);
@@ skipping 10 matching lines @@
         for (var typeParameter in ctx.typeParameters) {
           if (typeParameter.name == name) {
             return typeParameter;
           }
         }
       }
     }
     throw new StateError('Not found $kernelTypeParameter in $context');
   }
 }