Deprecate all diagnostics methods that use strings.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_mixin_application_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_mixin_application_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_mixin_application_builder.dart
index 1978c66b74564c33c379c86fc11a383c66458f2f..9868219f7539ad79f9ec604dac39609e1f4437f9 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_mixin_application_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_mixin_application_builder.dart
@@ -6,7 +6,7 @@ library fasta.kernel_mixin_application_builder;
 
 import 'package:kernel/ast.dart' show InterfaceType, Supertype;
 
-import '../errors.dart' show internalError;
+import '../deprecated_problems.dart' show deprecated_internalProblem;
 
 import '../util/relativize.dart' show relativizeUri;
 
@@ -40,10 +40,10 @@ class KernelMixinApplicationBuilder
         super(supertype, mixins, charOffset, fileUri);
 
   InterfaceType build(LibraryBuilder library) {
-    return internalError("Unsupported operation.");
+    return deprecated_internalProblem("Unsupported operation.");
   }
 
   Supertype buildSupertype(LibraryBuilder library) {
-    return internalError("Unsupported operation.");
+    return deprecated_internalProblem("Unsupported operation.");
   }
 }