Set return type of factory constructors properly.

pkg/front_end/testcases/rasta/issue_000002.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 import "package:expect/expect.dart" as exp;
 import "dart:typed_data" as typ;
 
 class Foo extends core::Object {
   final field dynamic value;
   constructor •(dynamic value) → void
     : self::Foo::value = value, super core::Object::•() {}
-  static factory fac(dynamic value) → dynamic {
+  static factory fac(dynamic value) → self::Foo {
     return new self::Foo::•(value);
   }
 }
 static field dynamic list = <dynamic>[1, 2, 3];
 static method main() → dynamic {
   exp::Expect::isTrue(typ::Uint8List::fromList(self::list).[](1).==(2));
   exp::Expect::isTrue(self::Foo::fac(10).value.==(10));
 }