Fix strong mode errors introduced by b62a37a36b2d01c3c27979a2cf5eada24afce035.

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 4265f317c75ec2d4cc0d9753547f27eb4af83cfb..32636457906343390b07876bb303d0d547a9eb37 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
@@ -52,8 +52,7 @@ import 'kernel_builder.dart' show
     KernelTypeBuilder,
     LibraryBuilder,
     MemberBuilder,
-    MetadataBuilder,
-    TypeBuilder;
+    MetadataBuilder;
 
 class KernelEnumBuilder extends SourceClassBuilder
     implements EnumBuilder<KernelTypeBuilder, InterfaceType> {
@@ -141,7 +140,7 @@ class KernelEnumBuilder extends SourceClassBuilder
     return enumBuilder;
   }
 
-  TypeBuilder get mixedInType => null;
+  KernelTypeBuilder get mixedInType => null;
 
   InterfaceType buildType(List<KernelTypeBuilder> arguments) {
     return cls.rawType;