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

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

Index: pkg/analyzer/lib/src/kernel/resynthesize.dart
diff --git a/pkg/analyzer/lib/src/kernel/resynthesize.dart b/pkg/analyzer/lib/src/kernel/resynthesize.dart
index d018091f77bad8ae246ecfbe77b349acd2738f43..78aebc6d0fe4aa3cb8d69a7e17c5cf11c38e340c 100644
--- a/pkg/analyzer/lib/src/kernel/resynthesize.dart
+++ b/pkg/analyzer/lib/src/kernel/resynthesize.dart
@@ -637,7 +637,7 @@ class _KernelLibraryResynthesizerContextImpl
     }
 
     // Search in the class.
-    if (parentElement is ClassElementImpl) {
+    if (parentElement is AbstractClassElementImpl) {
       if (isGetter) {
         return parentElement.getGetter(name.name) as ElementImpl;
       } else if (isSetter) {
@@ -662,7 +662,8 @@ class _KernelLibraryResynthesizerContextImpl
       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;