Fix type inference of PropertySet when the receiver is a generic class.

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..0611a871cbcb00421c7ff844fbdc34a2ce22fca3 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
@@ -1445,7 +1445,7 @@ class KernelPropertyAssign extends KernelComplexAssignmentWithReceiver {
     // To replicate analyzer behavior, we base type inference on the write
     // member.  TODO(paulberry): would it be better to use the read member when
     // doing compound assignment?
-    var writeContext = writeMember?.setterType;
+    var writeContext = inferrer.getSetterType(writeMember, receiverType);
     var inferredType = _inferRhs(inferrer, writeContext);
     inferrer.listener.propertyAssignExit(desugared, inferredType);
     return inferredType;