Implement override-based type inference for instance methods.

pkg/front_end/testcases/inference/infer_use_of_void.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 B {
   void f() {}
 }
 
 class C extends B {
-  f() {}
+  /*@topType=void*/ f() {}
 }
 
 var /*@topType=dynamic*/ x =
     new C(). /*info:USE_OF_VOID_RESULT*/ /*@target=C::f*/ f();
 
 main() {
   x;
 }