Support for modular mixin resolution.

pkg/kernel/binary.md

 This file describes the binary format of Dart Kernel.
 
 Notation
 --------
 Bitmasks are described with the syntax:
 ```scala
     Byte flags (flag1, flag2, ..., flagN)
 ```
 where 'flag<N>' is the N-th least significant bit,
 (so flag1 is the least significant bit).
@@ skipping 166 matching lines @@
   UriReference fileUri;
   List<Class> classes;
   List<Field> fields;
   List<Procedure> procedures;
 }
 
 abstract type Node {
   Byte tag;
 }
 
+enum ClassLevel { Type = 0, Hierarchy = 1, Mixin = 2, Body = 3, }
+
 // A class can be represented at one of three levels: type, hierarchy, or body.
 //
 // If the enclosing library is external, a class is either at type or
 // hierarchy level, depending on its isTypeLevel flag.
 // If the enclosing library is not external, a class is always at body level.
 //
 // See ClassLevel in ast.dart for the details of each loading level.
 
 abstract type Class extends Node {}
 
 type NormalClass extends Class {
   Byte tag = 2;
   FileOffset fileOffset;
-  Byte flags (isAbstract, isTypeLevel);
+  Byte flags (isAbstract, xx); // Where xx is index into ClassLevel
   StringReference name;
   // An absolute path URI to the .dart file from which the class was created.
   UriReference fileUri;
   List<Expression> annotations;
   List<TypeParameter> typeParameters;
   Option<InterfaceType> superClass;
   List<InterfaceType> implementedClasses;
   List<Field> fields;
   List<Constructor> constructors;
   List<Procedure> procedures;
@@ skipping 706 matching lines @@
 
 /* enum BaseClassKind { None, Exact, Subclass, Subtype, } */
 
 type InferredValue {
   ClassReference baseClass; // May be NullReference if kind = None.
   Byte kind; // Index into BaseClassKind.
   Byte valueBits; // See lib/type_propagation/type_propagation.dart
 }
 
 ```