Add mixin accessor.

sdk/lib/mirrors/mirrors.dart

Index: sdk/lib/mirrors/mirrors.dart
===================================================================
--- sdk/lib/mirrors/mirrors.dart	(revision 28620)
+++ sdk/lib/mirrors/mirrors.dart	(working copy)
@@ -764,6 +764,14 @@
   List<ClassMirror> get superinterfaces;
 
   /**
+   * The mixin of this class.
+   * If this class is the result of a mixin application of the
+   * form S with M, returns a class mirror on M.
+   * Otherwise returns a class mirror on [reflectee].

[ahe, 2013/10/15 07:00:38]

Otherwise returns this.

[gbracha, 2013/10/15 17:03:21]

That is a stronger specification. In principle, you could return a different
instance of ClassMirror. I deliberately left that open to the implementation.

[ahe, 2013/10/16 07:28:39]

Returning "this" is more accurate in case of subclasses of ClassMirror. This
might be a hypothetical point, as I think all subclasses of ClassMirrors are
going away. It also makes it clear what should happen if you have an original
declaration versus an instantiated type.

However, both these points apply to the class mirror on M as well. But they also
apply to [superclass] and [superinterfaces] above.

Can we somehow clarify this?

+   */
+  ClassMirror get mixin;
+
+  /**
    * An immutable map from names to mirrors for all members of
    * this type.
    *