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

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index 3cd869b03f61ae03a8232c95f5ca021e020124d3..8339dfe567b8e436469506abc9a2d821649ecbe1 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -616,6 +616,10 @@ class Class extends NamedNode {
   String name;
   bool isAbstract;
 
+  /// Whether this class is an enum.
+  @informative
+  bool isEnum = false;
+
   /// Whether this class is a synthetic implementation created for each
   /// mixed-in class. For example the following code:
   /// class Z extends A with B, C, D {}