Deprecate all diagnostics methods that use strings.

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 123f079e1ca8ec64d8f519215deac07fd636a8d3..abc88b20a63c08f78688f09b81d3855e1756aaa6 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 '../errors.dart' show internalError;
+import '../deprecated_problems.dart' show deprecated_internalProblem;
 
 import '../kernel/kernel_builder.dart'
     show MemberBuilder, KernelClassBuilder, KernelTypeBuilder, Scope;
@@ -52,10 +52,11 @@ class DillClassBuilder extends KernelClassBuilder {
   /// superclass.
   bool get isMixinApplication => cls.isMixinApplication;
 
-  KernelTypeBuilder get mixedInType => internalError("Not implemented.");
+  KernelTypeBuilder get mixedInType =>
+      deprecated_internalProblem("Not implemented.");
 
   void set mixedInType(KernelTypeBuilder mixin) {
-    internalError("Not implemented.");
+    deprecated_internalProblem("Not implemented.");
   }
 }