Implement type variables in mixin applications.

pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
index af46a61d45d6cf93606e0b2e84b60ac21ebeca97..4265f317c75ec2d4cc0d9753547f27eb4af83cfb 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_enum_builder.dart
@@ -52,7 +52,8 @@ import 'kernel_builder.dart' show
     KernelTypeBuilder,
     LibraryBuilder,
     MemberBuilder,
-    MetadataBuilder;
+    MetadataBuilder,
+    TypeBuilder;
 
 class KernelEnumBuilder extends SourceClassBuilder
     implements EnumBuilder<KernelTypeBuilder, InterfaceType> {
@@ -140,6 +141,8 @@ class KernelEnumBuilder extends SourceClassBuilder
     return enumBuilder;
   }
 
+  TypeBuilder get mixedInType => null;
+
   InterfaceType buildType(List<KernelTypeBuilder> arguments) {
     return cls.rawType;
   }