Implement override-based type inference for instance methods.

pkg/front_end/testcases/inference/unsafe_block_closure_inference_method_call_no_type_param.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 C {
-  double f(x) => 1.0;
+  double f(/*@topType=dynamic*/ x) => 1.0;
 }
 
 var /*@topType=double*/ v = new C(). /*@target=C::f*/ f(/*@returnType=int*/ () {
   return 1;
 });
 
 main() {
   v;
 }