Update doc-import to primary configured import, _goldens_io.dart (#151390)

There is a... restriction in dartdoc, such that a library which is
`@docImport`ed in one library, must be otherwise regularly imported in
some other library. Without this case, dartdoc gets a bit lost about
where the doc-imported library is.

Combine this restriction with analyzer's restriction that it doesn't
super duper handle configuration imports. Since `_goldens_io.dart` is
the "primary" import in all other files, and `_goldens_web.dart` is the
"configured" import, we must only doc-import `_goldens_io.dart`.

Author: srawlins

packages/flutter_test/lib/src/goldens.dart

@@ -4,7 +4,7 @@
 
 /// @docImport 'package:flutter/widgets.dart';
 ///
-/// @docImport '_goldens_web.dart';
+/// @docImport '_goldens_io.dart';
 /// @docImport 'binding.dart';
 /// @docImport 'matchers.dart';
 /// @docImport 'widget_tester.dart';