Move dart2jslib parts into separate libraries.

pkg/compiler/lib/src/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;
+library dart2js.script;
+
+import 'io/source_file.dart';
 
 class Script {
   final SourceFile file;
 
   /**
    * The readable URI from which this script was loaded.
    *
    * See [LibraryLoader] for terminology on URIs.
    */
   final Uri readableUri;
@@ skipping 14 matching lines @@
       {this.isSynthesized: false});
 
   String get text => (file == null) ? null : file.slowText();
   String get name => (file == null) ? null : file.filename;
 
   /// Creates a new [Script] with the same URIs, but new content ([file]).
   Script copyWithFile(SourceFile file) {
     return new Script(readableUri, resourceUri, file);
   }
 }