Implement override-based type inference for instance methods.

pkg/front_end/testcases/inference/future_then_conditional.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(self::MyFuture::T x) → 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 method test() → void {
   self::MyFuture<core::bool> f;
   asy::Future<core::int> t1 = f.{self::MyFuture::then}<core::int>((core::bool x) → asy::Future<core::int> async => x ?{core::int} 2 : await asy::Future::value<core::int>(3));
   asy::Future<core::int> t2 = f.{self::MyFuture::then}<core::int>((core::bool x) → asy::Future<core::int> async {
     return await x ?{core::Object} 2 : asy::Future::value<core::int>(3);
   });
   asy::Future<core::int> t5 = f.{self::MyFuture::then}<core::int>((core::bool x) → core::Object => x ?{core::Object} 2 : asy::Future::value<core::int>(3));
   asy::Future<core::int> t6 = f.{self::MyFuture::then}<core::int>((core::bool x) → asy::FutureOr<core::int> {
     return x ?{core::Object} 2 : asy::Future::value<core::int>(3);
   });
 }
 static method main() → dynamic {}