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 179 matching lines @@
 // 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 of class level

[Kevin Millikin (Google), 2017/02/09 10:44:34]

This should be spelled out explicitly here, even if it's redundant with
ast.dart.

The comment above also mentions only three levels, ast.dart includes five, and
it's ambiguous which index we're talking about.  I guess I'd rather have
something like:

enum ClassLevel { Type = 0, Hierarchy = 1, Mixin = 2, Body = 3, }

[asgerf, 2017/02/13 12:43:35]

Done.

   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
 }
 
 ```