Implement override-based type inference for instance methods.

pkg/front_end/testcases/inference/future_then_upwards_2.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(T x) {}
-  dynamic noSuchMethod(invocation);
+  dynamic noSuchMethod(/*@topType=Invocation*/ invocation);
   MyFuture<S> then<S>(FutureOr<S> f(T x), {Function onError}) => null;
 }
 
 void main() {
   var /*@type=MyFuture<double>*/ f =
       foo(). /*@typeArgs=double*/ /*@target=MyFuture::then*/ then(
           /*@returnType=double*/ (/*@type=dynamic*/ _) => 2.3);
   MyFuture<int> f2 = /*error:INVALID_ASSIGNMENT*/ f;
 
   // The unnecessary cast is to illustrate that we inferred <double> for
   // the generic type args, even though we had a return type context.
   MyFuture<num> f3 = /*info:UNNECESSARY_CAST*/ foo()
           . /*@typeArgs=double*/ /*@target=MyFuture::then*/ then(
               /*@returnType=double*/ (/*@type=dynamic*/ _) => 2.3)
       as MyFuture<double>;
 }
 
 MyFuture foo() => new MyFuture<int>.value(1);