Handle deferred access of unserialized code.

tests/compiler/dart2js/serialization/test_data.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library dart2js.serialization_test_data;
 
 const List<Test> TESTS = const <Test>[
   const Test('Empty program', const {
     'main.dart': 'main() {}'
   }),
@@ skipping 339 matching lines @@
 class S {}
 class M {}
 class C1 extends S with M {}
 class C2 extends S with M {}
 main() {
   new C1();
   new C2();
 }
 ''',
   }),
+
+  const Test('Deferred loading', const {
+    'main.dart': '''
+import 'a.dart' deferred as lib;
+main() {
+  lib.foo();
+}
+''',
+    'a.dart': '''
+void foo() {}
+''',
+  }),
 ];
 
 class Test {
   final String name;
   final Map sourceFiles;
   final Map preserializedSourceFiles;
   final Map unserializedSourceFiles;
   final int expectedErrorCount;
   final int expectedWarningCount;
   final int expectedHintCount;
   final int expectedInfoCount;
 
   const Test(
       this.name,
       this.sourceFiles,
       {this.preserializedSourceFiles,
       this.unserializedSourceFiles,
       this.expectedErrorCount: 0,
       this.expectedWarningCount: 0,
       this.expectedHintCount: 0,
       this.expectedInfoCount: 0});
 }