Issue 25686. Add 'var' for untyped unimplemented fields.

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index 948d8e3e9a6cc28d1ab23f2067e7a02aeda65a28..1e148de7e269f2d1f67f0bc0ce03448d6a8a299d 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -1249,7 +1249,7 @@ class FixProcessor {
           // add field
           addEolIfNotFirst();
           sb.append(utils.getIndent(1));
-          _appendType(sb, element.type.returnType);
+          _appendType(sb, element.type.returnType, orVar: true);
           sb.append(element.name);
           sb.append(';');
           sb.append(eol);