Implement override-based type inference for instance methods.

pkg/front_end/testcases/inference/future_union_downwards.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
 
 class MyFuture<T extends core::Object> extends core::Object implements asy::Future<self::MyFuture::T> {
   constructor •() → void
     : super core::Object::•() {}
   constructor value([dynamic x = null]) → void
     : super core::Object::•() {}
-  abstract method noSuchMethod(dynamic invocation) → dynamic;
+  abstract method noSuchMethod(core::Invocation invocation) → dynamic;
   method then<S extends core::Object>((self::MyFuture::T) → asy::FutureOr<self::MyFuture::then::S> f, {core::Function onError = null}) → self::MyFuture<self::MyFuture::then::S>
     return null;
 }
 static field self::MyFuture<dynamic> f;
 static field asy::Future<core::int> t1 = self::f.{self::MyFuture::then}<core::int>((dynamic _) → asy::Future<core::int> => asy::Future::value<core::int>("hi"));
 static field asy::Future<core::List<core::int>> t2 = self::f.{self::MyFuture::then}<core::List<core::int>>((dynamic _) → core::List<core::int> => <core::int>[3]);
 static method g2() → asy::Future<core::List<core::int>> async {
   return <core::int>[3];
 }
 static method g3() → asy::Future<core::List<core::int>> async {
   return asy::Future::value<core::List<core::int>>(<core::int>[3]);
 }
 static method main() → dynamic {}