Deprecate all diagnostics methods that use strings.

pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart

Index: pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart
diff --git a/pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart b/pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart
index f837e4f50ad2ac1e08556f5e211f9eccf8096b7d..3e79c0f86d40bb6b93e5f99a04bfe87519b9987d 100644
--- a/pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart
+++ b/pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart
@@ -200,7 +200,7 @@ class ValidatingInstrumentation implements Instrumentation {
 
   String _formatProblem(
       Uri uri, int offset, String desc, StackTrace stackTrace) {
-    return format(
+    return deprecated_format(
         uri, offset, '$desc${stackTrace == null ? '' : '\n$stackTrace'}');
   }