Implement override-based type inference for instance methods.

pkg/front_end/testcases/inference/type_cast.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<T> {}
 
 class B<T> extends A<T> {
-  foo() {}
+  /*@topType=dynamic*/ foo() {}
 }
 
 A<num> a = new B<int>();
 var /*@topType=B<int>*/ b = (a as B<int>);
 
 main() {
   A<num> a = new B<int>();
   var /*@type=B<int>*/ b = (a as B<int>);
 }