Deprecate all diagnostics methods that use strings.

pkg/front_end/lib/src/fasta/kernel/kernel_mixin_application_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_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;
 
 import 'kernel_builder.dart'
     show
         KernelLibraryBuilder,
         KernelTypeBuilder,
         LibraryBuilder,
         MixinApplicationBuilder,
         TypeVariableBuilder;
@@ skipping 13 matching lines @@
 
   String subclassName;
 
   KernelMixinApplicationBuilder(KernelTypeBuilder supertype,
       List<KernelTypeBuilder> mixins, this.library, int charOffset, Uri fileUri)
       : charOffset = charOffset,
         relativeFileUri = relativizeUri(fileUri),
         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.");
   }
 }