Add a clarifying comment explaining why UnknownType.accept() doesn't throw.

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

Index: pkg/front_end/lib/src/fasta/type_inference/type_schema.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_schema.dart b/pkg/front_end/lib/src/fasta/type_inference/type_schema.dart
index f9c7f77b00f66911155db4fa566b74c65911d4ce..5268f62d08a7fe41c855b3e687cfe6c78730b768 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_schema.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_schema.dart
@@ -61,6 +61,11 @@ class UnknownType extends DartType {
     if (v is TypeSchemaVisitor) {
       return v.visitUnknownType(this);
     } else {
+      // Note: in principle it seems like this should throw, since any visitor
+      // that operates on a type schema ought to inherit from TypeSchemaVisitor.
+      // However, that would make it impossible to use toString() on any type
+      // schema, since toString() uses the kernel's Printer visitor, which can't
+      // possibly inherit from TypeSchemaVisitor since it's inside kernel.
       return v.defaultDartType(this);
     }
   }