Compute correct return type for factory methods.

pkg/front_end/testcases/inference/downwards_inference_yield_yield_star.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 foo() → asy::Stream<core::List<core::int>> /* originally async* */ {
   dynamic :controller;
   dynamic :async_op_then;
   dynamic :async_op_error;
   dynamic :await_jump_var = 0;
   dynamic :await_ctx_var;
   dynamic :saved_try_context_var0;
   dynamic :saved_try_context_var1;
@@ skipping 54 matching lines @@
         :iterator.isYieldEach = true;
         :iterator._current = core::List::_internal<core::Map<core::int, core::int>>();
         [yield] true;
       }
     }
     return false;
   }
   return new core::_SyncIterable::•(:sync_op);
 }
 static method main() → dynamic {}