Implement override-based inference of instance fields.

pkg/front_end/testcases/inference/infer_from_rhs_only_if_it_wont_conflict_with_overridden_fields.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 {
   var /*@topType=dynamic*/ x;
 }
 
 class B implements A {
   var /*@topType=dynamic*/ x = 2;
 }
 
 foo() {
   String y = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
   int z = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
 }
+
+main() {}