Enable top level inference of instance property gets/sets.

pkg/front_end/testcases/inference_new/infer_instance_accessor_ref.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 f;
+  B b;
 }
 
-A a = new A();
-var /*@topType=dynamic*/ b =
-    (/*error:TOP_LEVEL_UNSUPPORTED*/ a. /*@target=A::f*/ f = 1);
-var /*@topType=int*/ c = 0;
+class B {
+  C get c => null;
+  void set(C value) {}
+}
 
-main() {
-  a;
-  b;
-  c;
-}
+class C {}
+
+class D extends C {}
+
+var /*@topType=A*/ a = new A();
+var /*@topType=C*/ x = a. /*@target=A::b*/ b. /*@target=B::c*/ c;
+var /*@topType=dynamic*/ y = a. /*@target=A::b*/ b.c ??= new D();
+
+main() {}