Make the test's dart:core import dart:async in tests.

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

Index: pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/core.dart
diff --git a/pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/core.dart b/pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/core.dart
index a7fa6579a1036ac968fc3e7b3437d698ffd23640..94b936c5aa578ed0be04653ab2609e36f733c154 100644
--- a/pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/core.dart
+++ b/pkg/analyzer_cli/test/data/package_with_embedder_yaml/lib/core.dart
@@ -4,6 +4,8 @@
 
 library dart.core;
 
+import 'dart:async';
+
 class Object {
   bool operator ==(other) => identical(this, other);
   String toString() => 'a string';