Avoid a downcast error

The field that we use to hold redirecting constructors is a Kernel
field, and not a ShadowField.

Fixes https://github.com/dart-lang/sdk/issues/34715

Change-Id: Iea2ce41f0229a7b9c2354bf3c6c3b586a3ffec33
Reviewed-on: https://dart-review.googlesource.com/c/78540
Reviewed-by: Peter von der Ahé <ahe@google.com>
Reviewed-by: Daniel Hillerström <hillerstrom@google.com>
Commit-Queue: Kevin Millikin <kmillikin@google.com>

pkg/front_end/lib/src/fasta/type_inference/type_inference_engine.dart

@@ -8,6 +8,7 @@ import 'package:kernel/ast.dart'
         DartType,
         DartTypeVisitor,
         DynamicType,
+        Field,
         FunctionType,
         InterfaceType,
         TypeParameter,
@@ -278,9 +279,9 @@ abstract class TypeInferenceEngine {
 
   void inferInitializingFormal(VariableDeclaration formal, Constructor parent) {
     if (formal.type == null) {
-      for (ShadowField field in parent.enclosingClass.fields) {
+      for (Field field in parent.enclosingClass.fields) {
         if (field.name.name == formal.name) {
-          if (field.inferenceNode != null) {
+          if (field is ShadowField && field.inferenceNode != null) {
             field.inferenceNode.resolve();
           }
           formal.type = field.type;