Mark normal classes that were originally mixin applications

To implement dart:mirrors correctly, a backend like the VM needs to
know that a class was originally a mixin application.  Use the
`isSyntheticMixinImplementation` flag which was already there and
ignored by the VM.

Now the property is:

- if `isSyntheticMixinImplementation` is set on a class with a
  mixed-in type, then it was originally an anonymous mixin application

- if `isSyntheticMixinImplementation` is set on a normal class then it
  was originally a mixin application of some kind and the mixed-in
  type can be found as the last impelemented type

Bug: https://github.com/dart-lang/sdk/issues/33240
Change-Id: I004adc6bfe08e583efba8e511076a6c603c0c687
Reviewed-on: https://dart-review.googlesource.com/56760
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Kevin Millikin <kmillikin@google.com>

pkg/kernel/binary.md

@@ -286,7 +286,8 @@ type Class extends Node {
   UriReference fileUri;
   FileOffset fileOffset;
   FileOffset fileEndOffset;
-  Byte flags (isAbstract, isEnum, xx); // Where xx is index into ClassLevel
+  Byte flags (isAbstract, isEnum, isSyntheticMixinApplication,
+              xx); // Where xx is index into ClassLevel
   StringReference name;
   List<Expression> annotations;
   List<TypeParameter> typeParameters;

pkg/kernel/lib/transformations/mixin_full_resolution.dart

@@ -232,11 +232,13 @@ class MixinFullResolution {
       }
     }
 
-    // This class implements the mixin type.
+    // This class implements the mixin type.  Implementations can rely on it
+    // occurring last.
     class_.implementedTypes.add(class_.mixedInType);
 
     // This class is now a normal class.
     class_.mixedInType = null;
+    class_.isSyntheticMixinImplementation = true;
   }
 
   Constructor buildForwardingConstructor(