Support serialization of ambiguous elements.

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 bf8ecf8330739ef172d0f0389aacefb3bbe94879..cfb2d08e70dbfd9d9ddd2b095a70f0943c64d08a 100644
--- a/tests/compiler/dart2js/serialization/test_data.dart
+++ b/tests/compiler/dart2js/serialization/test_data.dart
@@ -717,6 +717,21 @@ export 'main.dart';
 
 main() {}
 '''}),
+
+  const Test('Ambiguous elements', const {},
+      preserializedSourceFiles: const {
+        'main.dart': '''
+import 'a.dart';
+import 'b.dart';
+
+main() => new foo();
+''',
+        'a.dart': '''
+var foo;
+''',
+        'b.dart': '''
+var foo;
+''',}),
 ];
 
 class Test {