Support references to unserialized libraries.

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 252e6653fc791d7233bafbee988d20f670b3af5a..987372af27afb706d6e9d408bf96fb4676daaee1 100644
--- a/tests/compiler/dart2js/serialization/test_data.dart
+++ b/tests/compiler/dart2js/serialization/test_data.dart
@@ -327,12 +327,30 @@ main() => m();
 m() => print(#main);
 ''',
   }),
+
+  const Test('Indirect unserialized library', const {
+    'main.dart': '''
+import 'a.dart';
+main() => foo();
+''',
+  }, preserializedSourceFiles: const {
+    'a.dart': '''
+import 'memory:b.dart';
+foo() => bar();
+''',
+  }, unserializedSourceFiles: const {
+    'b.dart': '''
+import 'memory:a.dart';
+bar() => foo();
+''',
+  }),
 ];
 
 class Test {
   final String name;
   final Map sourceFiles;
   final Map preserializedSourceFiles;
+  final Map unserializedSourceFiles;
   final int expectedErrorCount;
   final int expectedWarningCount;
   final int expectedHintCount;
@@ -342,6 +360,7 @@ class Test {
       this.name,
       this.sourceFiles,
       {this.preserializedSourceFiles,
+      this.unserializedSourceFiles,
       this.expectedErrorCount: 0,
       this.expectedWarningCount: 0,
       this.expectedHintCount: 0,