Support for modular mixin resolution.

pkg/kernel/binary.md

Index: pkg/kernel/binary.md
diff --git a/pkg/kernel/binary.md b/pkg/kernel/binary.md
index a287dd6be83b9b2884a61d04c12ab347a3fb7af5..f08f239fb8d94937eacb16588c1ec019ecbc8c94 100644
--- a/pkg/kernel/binary.md
+++ b/pkg/kernel/binary.md
@@ -197,7 +197,7 @@ 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;