Remove deprecated_internalProblem.

pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart b/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
index abc88b20a63c08f78688f09b81d3855e1756aaa6..3af2756416a417f509542bbe7b075f516a13ca6b 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
@@ -6,7 +6,7 @@ library fasta.dill_class_builder;
 
 import 'package:kernel/ast.dart' show Class, Member;
 
-import '../deprecated_problems.dart' show deprecated_internalProblem;
+import '../problems.dart' show unimplemented;
 
 import '../kernel/kernel_builder.dart'
     show MemberBuilder, KernelClassBuilder, KernelTypeBuilder, Scope;
@@ -52,11 +52,10 @@ class DillClassBuilder extends KernelClassBuilder {
   /// superclass.
   bool get isMixinApplication => cls.isMixinApplication;
 
-  KernelTypeBuilder get mixedInType =>
-      deprecated_internalProblem("Not implemented.");
+  KernelTypeBuilder get mixedInType => unimplemented("mixedInType", -1, null);
 
   void set mixedInType(KernelTypeBuilder mixin) {
-    deprecated_internalProblem("Not implemented.");
+    unimplemented("mixedInType=", -1, null);
   }
 }