Serialize this-types in NativeBehavior separately.

tests/compiler/dart2js/memory_source_file_helper.dart

 // Copyright (c) 2013, 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.test.memory_source_file_helper;
 
 import 'dart:async' show Future;
 export 'dart:io' show Platform;
 
 export 'package:compiler/src/apiimpl.dart'
@@ skipping 15 matching lines @@
   Map<String, String> memorySourceFiles;
 
   MemorySourceFileProvider(Map<String, String> this.memorySourceFiles);
 
   Future<String> readStringFromUri(Uri resourceUri) {
     if (resourceUri.scheme != 'memory') {
       return super.readStringFromUri(resourceUri);
     }
     String source = memorySourceFiles[resourceUri.path];
     if (source == null) {
-      return new Future.error(new Exception('No such file $resourceUri'));
+      return new Future.error(new Exception(
+          'No such memory file $resourceUri in ${memorySourceFiles.keys}'));
     }
     this.sourceFiles[resourceUri] =
         new StringSourceFile.fromUri(resourceUri, source);
     return new Future.value(source);
   }
 
   Future<String> call(Uri resourceUri) => readStringFromUri(resourceUri);
 
   SourceFile getSourceFile(Uri resourceUri) {
     if (resourceUri.scheme != 'memory') {
       return super.getSourceFile(resourceUri);
     }
     String source = memorySourceFiles[resourceUri.path];
     if (source == null) {
-      throw new Exception('No such file $resourceUri');
+      throw new Exception(
+          'No such memory file $resourceUri in ${memorySourceFiles.keys}');
     }
     return new StringSourceFile.fromUri(resourceUri, source);
   }
 
   @override
   Future readFromUri(Uri resourceUri) => readStringFromUri(resourceUri);
 }