Resynthesize mixins from Kernel.

pkg/kernel/lib/binary/ast_to_binary.dart

Index: pkg/kernel/lib/binary/ast_to_binary.dart
diff --git a/pkg/kernel/lib/binary/ast_to_binary.dart b/pkg/kernel/lib/binary/ast_to_binary.dart
index f0c2efd4299e12166ae9c8bab8bb86e9838aa7eb..b1a55e58ba29b308fea6ec75e91bdc2ad28206b4 100644
--- a/pkg/kernel/lib/binary/ast_to_binary.dart
+++ b/pkg/kernel/lib/binary/ast_to_binary.dart
@@ -363,14 +363,18 @@ class BinaryPrinter extends Visitor {
     writeList(annotations, writeAnnotation);
   }
 
-  int _encodeClassFlags(bool isAbstract, ClassLevel level) {
-    int abstactFlag = isAbstract ? 1 : 0;
-    int levelFlags = (level.index - 1) << 1;
-    return abstactFlag | levelFlags;
+  int _encodeClassFlags(
+      bool isAbstract, bool isSyntheticMixinImplementation, ClassLevel level) {
+    int abstractFlag = isAbstract ? 1 : 0;
+    int isSyntheticMixinImplementationFlag =
+        isSyntheticMixinImplementation ? 2 : 0;
+    int levelFlags = (level.index - 1) << 2;
+    return abstractFlag | isSyntheticMixinImplementationFlag | levelFlags;
   }
 
   visitClass(Class node) {
-    int flags = _encodeClassFlags(node.isAbstract, node.level);
+    int flags = _encodeClassFlags(
+        node.isAbstract, node.isSyntheticMixinImplementation, node.level);
     if (node.canonicalName == null) {
       throw 'Missing canonical name for $node';
     }