Enable top level inference of instance property gets/sets.

pkg/front_end/testcases/inference/infer_assign_to_property_custom.dart

-// Copyright (c) 2017, 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.
-
-/*@testedFeatures=inference*/
-library test;
-
-class A {
-  int operator +(other) => 1;
-  double operator -(other) => 2.0;
-}
-
-class B {
-  A a;
-}
-
-var /*@topType=dynamic*/ v_prefix_pp =
-    (++new B(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a);
-var /*@topType=dynamic*/ v_prefix_mm =
-    (--new B(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a);
-var /*@topType=dynamic*/ v_postfix_pp =
-    (new B(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a++);
-var /*@topType=dynamic*/ v_postfix_mm =
-    (new B(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=B::a*/ a--);
-
-main() {}