Handle missing imports/exports/parts.

sdk/lib/_internal/compiler/implementation/script.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 class Script {
-  // TODO(kasperl): Once MockFile in tests/compiler/dart2js/parser_helper.dart
-  // implements SourceFile, we should be able to type the [file] field as
-  // such.
-  final file;
+  final SourceFile file;
 
   /**
    * The readable URI from which this script was loaded.
    *
    * See [LibraryLoader] for terminology on URIs.
    */
   final Uri uri;
 
   Script(this.uri, this.file);
 
   String get text => (file == null) ? null : file.text;
   String get name => (file == null) ? null : file.filename;
 }