Enable top level inference of instance property gets/sets.

pkg/front_end/testcases/inference_new/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=int*/ v_prefix_pp = (++new B(). /*@target=B::a*/ a);
+var /*@topType=double*/ v_prefix_mm = (--new B(). /*@target=B::a*/ a);
+var /*@topType=A*/ v_postfix_pp = (new B(). /*@target=B::a*/ a++);
+var /*@topType=A*/ v_postfix_mm = (new B(). /*@target=B::a*/ a--);
+
+main() {}