Implement type inference of getters/setters based on inheritance.

pkg/front_end/testcases/inference/conflicts_can_happen2.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 I1 {
   int x;
 }
@@ skipping 13 matching lines @@
 
 class B {
   final I2 a = null;
 }
 
 class C1 implements A, B {
   I3 get a => null;
 }
 
 class C2 implements A, B {
-  /*error:INVALID_METHOD_OVERRIDE*/ get a => null;
+  /*error:INVALID_METHOD_OVERRIDE*/ get /*@topType=dynamic*/ a => null;
 }
 
 main() {}