Make the kernel API for promoted type parameter bounds less error-prone.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index c7b4bce4de4bccf46e074ca3f043787a7e098aac..926979e56836d5e7771ec8294fecf3d5cef11444 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -1937,7 +1937,7 @@ class KernelTypeInferrer extends TypeInferrerImpl {
   DartType resolveTypeParameter(DartType type) {
     DartType resolveOneStep(DartType type) {
       if (type is TypeParameterType) {
-        return type.bound ?? type.parameter.bound;
+        return type.bound;
       } else {
         return null;
       }