Deprecate all diagnostics methods that use strings.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
index 7a52861b203c9c7790d03b8b286ef9cc3b74cf6d..63016ad9e54ca989b5e72aa6cb56cbcab26aa985 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
@@ -25,7 +25,7 @@ import 'package:front_end/src/fasta/type_inference/type_inference_listener.dart'
 import 'package:kernel/ast.dart'
     show DartType, Expression, Field, Name, NullLiteral;
 
-import '../errors.dart' show internalError;
+import '../deprecated_problems.dart' show deprecated_internalProblem;
 
 import 'kernel_builder.dart'
     show Builder, FieldBuilder, KernelTypeBuilder, MetadataBuilder;
@@ -52,7 +52,8 @@ class KernelFieldBuilder extends FieldBuilder<Expression> {
 
   void set initializer(Expression value) {
     if (!hasInitializer && value is! NullLiteral && !isConst && !isFinal) {
-      internalError("Attempt to set initializer on field without initializer.");
+      deprecated_internalProblem(
+          "Attempt to set initializer on field without initializer.");
     }
     field.initializer = value..parent = field;
   }