Add library mapping.

dart/utils/compiler/build_helper.dart

 // Copyright (c) 2012, 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.
 
 #import('dart:io');
 #import('dart:uri');
 
 main() {
   List<String> arguments = new Options().arguments;
   Uri uri = new Uri(scheme: 'file', path: '${arguments[0]}/');
@@ skipping 25 matching lines @@
 buildScript(Uri uri) {
   return """
 
 #import('dart:io');
 
 #import('${uri.resolve('../../lib/compiler/implementation/dart2js.dart').path}');
 
 class Helper {
   void run() {
     try {
-      List<String> argv =
-        ['--library-root=${uri.resolve('../../lib/compiler/implementation/lib').path}'];
+      List<String> argv = ['--library-root=${uri.resolve('../../').path}'];
       argv.addAll(new Options().arguments);
       compile(argv);
     } catch (var exception, var trace) {
       try {
         print('Internal error: \$exception');
       } catch (var ignored) {
         print('Internal error: error while printing exception');
       }
       try {
         print(trace);
       } finally {
         exit(253); // 253 is recognized as a crash by our test scripts.
       }
     }
   }
 }
 
 void main() {
   new Helper().run();
 }
 """;
 }