Compute correct return type for factory methods.

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

 library test;
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
 
 abstract class MyStream<T extends core::Object> extends asy::Stream<self::MyStream::T> {
-  static factory •<T extends core::Object>() → dynamic
+  static factory •<T extends core::Object>() → self::MyStream<self::MyStream::•::T>
     return null;
 }
 static method F<T extends core::Object>() → self::F::T
   return null;
 static method f() → asy::Future<dynamic> /* originally async */ {
   final asy::Completer<asy::FutureOr<dynamic>> :completer = asy::Completer::sync<asy::FutureOr<dynamic>>();
   asy::FutureOr<dynamic> :return_value;
   dynamic :async_op_then;
   dynamic :async_op_error;
   dynamic :await_jump_var = 0;
@@ skipping 202 matching lines @@
       return;
     }
     on dynamic catch(dynamic :exception, dynamic :stack_trace) {
       :completer.completeError(:exception, :stack_trace);
     }
   :async_op_then = asy::_asyncThenWrapperHelper(:async_op);
   :async_op_error = asy::_asyncErrorWrapperHelper(:async_op);
   asy::Future::microtask<dynamic>(:async_op);
   return :completer.future;
 }