Support for modular mixin resolution.

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 7535f619c97c2ae64989498e1750f5f2259b1ee5..f4eb825d4f37340c3bc843281b646948623f2da9 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -274,6 +274,18 @@ enum ClassLevel {
   /// members needed to detect override constraints.
   Hierarchy,
 
+  /// All instance members of the class have their body loaded, and their
+  /// annotations are present.
+  ///
+  /// All supertypes of this class are at [Hierarchy] level or higher.
+  ///
+  /// If this class is a mixin application, then its mixin is loaded at [Mixin]
+  /// level or higher.
+  ///
+  /// This level exists so the contents of a mixin can be cloned into a
+  /// mixin application.
+  Mixin,
+
   /// All members of the class are fully loaded and are in the correct order.
   ///
   /// Annotations are present on classes and members.