Handle deferred access of unserialized code.

tests/compiler/dart2js/serialization/test_data.dart

Index: tests/compiler/dart2js/serialization/test_data.dart
diff --git a/tests/compiler/dart2js/serialization/test_data.dart b/tests/compiler/dart2js/serialization/test_data.dart
index 54c958b841f341aa6f081f52dd155855a2ea8e02..51670bf5669e8d04c8aea022dd2bba77f72b91db 100644
--- a/tests/compiler/dart2js/serialization/test_data.dart
+++ b/tests/compiler/dart2js/serialization/test_data.dart
@@ -357,6 +357,18 @@ main() {
 }
 ''',
   }),
+
+  const Test('Deferred loading', const {
+    'main.dart': '''
+import 'a.dart' deferred as lib;
+main() {
+  lib.foo();
+}
+''',
+    'a.dart': '''
+void foo() {}
+''',
+  }),
 ];
 
 class Test {