Support serialization of ambiguous elements.

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>[
   // These tests are very long-running and put here first to compile them on
   // their own tests.
   const Test('Disable tree shaking through reflection', const {
@@ skipping 699 matching lines @@
 '''}),
 
   const Test('Metadata on exports', const {},
       preserializedSourceFiles: const {
         'main.dart': '''
 @deprecated
 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 {
   final String name;
   final Map sourceFiles;
   final Map preserializedSourceFiles;
   final Map unserializedSourceFiles;
   final int expectedErrorCount;
   final int expectedWarningCount;
   final int expectedHintCount;
   final int expectedInfoCount;
   final bool checkedMode;
 
   const Test(
       this.name,
       this.sourceFiles,
       {this.preserializedSourceFiles,
       this.unserializedSourceFiles,
       this.expectedErrorCount: 0,
       this.expectedWarningCount: 0,
       this.expectedHintCount: 0,
       this.expectedInfoCount: 0,
       this.checkedMode: false});
 }