Avoid generating members for classes only for needed rti.

tests/language/regress_14105_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Regression test for Issue 14105.
+
+typedef UsedAsFieldType();
+
+class ClassOnlyForRti {
+  UsedAsFieldType field;
+}
+
+class A<T> {
+  var field;
+}
+
+use(a) => a.field = "";
+
+var useFieldSetter = use;
+
+main() {
+  var a = new A<ClassOnlyForRti>();
+  useFieldSetter(a);
+  print(a is A<int>);
+}