CLI support for embedders (#25380).

pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/async.dart

Index: pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/async.dart
diff --git a/tests/language/export_not_shadowed_by_prefix_helper2.dart b/pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/async.dart
similarity index 73%
copy from tests/language/export_not_shadowed_by_prefix_helper2.dart
copy to pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/async.dart
index ae2dca5fcf0a4842b29e20d7652b35b6bfd65cfc..3b69a2a4dca3f7b1454b5127c2e71c561a8ff496 100644
--- a/tests/language/export_not_shadowed_by_prefix_helper2.dart
+++ b/pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/async.dart
@@ -2,8 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-void f() {
-  f_called = true;
-}
+library dart.async;
 
-bool f_called = false;
+abstract class Future<T> {}
+
+abstract class Stream<T> {}