Fix strong mode errors introduced by b62a37a36b2d01c3c27979a2cf5eada24afce035.

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.kernel_enum_builder;
 
 import 'package:kernel/ast.dart' show
     Arguments,
     AsyncMarker,
     Class,
@@ skipping 34 matching lines @@
     FormalParameterBuilder,
     KernelConstructorBuilder,
     KernelFieldBuilder,
     KernelFormalParameterBuilder,
     KernelLibraryBuilder,
     KernelNamedTypeBuilder,
     KernelProcedureBuilder,
     KernelTypeBuilder,
     LibraryBuilder,
     MemberBuilder,
-    MetadataBuilder,
-    TypeBuilder;
+    MetadataBuilder;
 
 class KernelEnumBuilder extends SourceClassBuilder
     implements EnumBuilder<KernelTypeBuilder, InterfaceType> {
   final List<String> constants;
 
   final MapLiteral toStringMap;
 
   final KernelTypeBuilder intType;
 
   final KernelTypeBuilder stringType;
@@ skipping 67 matching lines @@
         parent, charOffset);
     // TODO(sigmund): dynamic should be `covariant MemberBuilder`.
     members.forEach((String name, dynamic b) {
       MemberBuilder builder = b;
       builder.parent = enumBuilder;
     });
     selfType.builder = enumBuilder;
     return enumBuilder;
   }
 
-  TypeBuilder get mixedInType => null;
+  KernelTypeBuilder get mixedInType => null;
 
   InterfaceType buildType(List<KernelTypeBuilder> arguments) {
     return cls.rawType;
   }
 
   Class build(KernelLibraryBuilder libraryBuilder) {
     if (constants.isEmpty) {
       libraryBuilder.addCompileTimeError(
           -1, "An enum declaration can't be empty.");
     }
@@ skipping 27 matching lines @@
       Arguments arguments =
           new Arguments(<Expression>[new IntLiteral(index++)]);
       field.initializer =
           new ConstructorInvocation(constructor, arguments, isConst: true);
     }
     return super.build(libraryBuilder);
   }
 
   Builder findConstructorOrFactory(String name) => null;
 }