Implement override-based type inference for instance methods.

pkg/front_end/testcases/inference/future_union_downwards.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;
 
 import 'dart:async';
 
 class MyFuture<T> implements Future<T> {
   MyFuture() {}
   MyFuture.value([x]) {}
-  dynamic noSuchMethod(invocation);
+  dynamic noSuchMethod(/*@topType=Invocation*/ invocation);
   MyFuture<S> then<S>(FutureOr<S> f(T x), {Function onError}) => null;
 }
 
 MyFuture f;
 // Instantiates Future<int>
 Future<int> t1 = f. /*@typeArgs=int*/ /*@target=MyFuture::then*/ then(
     /*@returnType=Future<int>*/ (/*@type=dynamic*/ _) =>
         new /*@typeArgs=int*/ Future.value('hi'));
 
 // Instantiates List<int>
 Future<List<int>> t2 =
     f. /*@typeArgs=List<int>*/ /*@target=MyFuture::then*/ then(
         /*@returnType=List<int>*/ (/*@type=dynamic*/ _) => /*@typeArgs=int*/ [
               3
             ]);
 Future<List<int>> g2() async {
   return /*@typeArgs=int*/ [3];
 }
 
 Future<List<int>> g3() async {
   return new /*@typeArgs=List<int>*/ Future.value(
       /*@typeArgs=int*/ [3]);
 }
 
 main() {}